DS spends all his time with GF and she doesn’t speak to us

Oh, that's the solution! If only someone had explained to my autistic/ SM child that they were being absolutely ridiculous and rude when they were 6. Then they wouldn't have had a nervous breakdown at 17 and now be 25, still living at home on high level PIP and with an adult social worker and a carer to help them leave the house.

Sod, if only I had known.

There's being shy and then there's being absolutely ridiculous and rude. No one should act like this beyond the age of 6. How the hell will she cope in the workplace?

Unless the OP comes back with a drip feed that something had happened to make her angry, it's very unlikely.
Autistic people can struggle to process, describe and respond to their emotions. They don't get angry with no stimulus...
Edited as posted too soon...

She does not appear to be experiencing the emotion of anger, just adopting a facial expression that would indicate anger.
Your SIL, if she has a personality disorder will actually be feeling the emotion of anger, however unjustified.

Do we know this, though? The gf might or might not be feeling anger, either may be true. We have no way of telling.

The difference is that the GF is demonstrating a lack of functionality in her non verbal communication. She does not appear to be experiencing the emotion of anger, just adopting a facial expression that would indicate anger.
Your SIL, if she has a personality disorder will actually be feeling the emotion of anger, however unjustified.
Whilst this obviously is not enough to diagnose autism, it does evidence impaired social communication which is one of the diagnostic criteria.

It could be selective mutism (a child in my DS’ class has it) but I find it a little difficult to believe that she wouldn’t have shared this with your DS and that he would simply think she is shy if that is the case. Not impossible but pretty unlikely. As for appearing angry, the child I know that has SM does give the appearance of being angry, bless her. Her mother is lovely and I’ve been to their house a few times. I say hello to the child but she does just give an angry look. What is clear is that she is neurodiverse. Her mother only explained recently that it was SM and it makes complete sense. This child is in primary school though. It’s hard to imagine something as debilitating as SM wouldn’t be picked up at all by age 16. I suppose it’s impossible for those on the thread to know.

Agree with this @Eaglemom . maybe she does have selective mutism- but maybe she doesn’t. I think you are very sensitive around this issue for personal reasons and perhaps need to be a little more objective/accepting of others experiences/opinions.

I thinks it’s absolutely fine to mention selective mutism as a possibility for OP to consider or share knowledge about how it can present. It is absolutely not right to state it as an almost certainty that this young women has selective mutism and suggest there is little possibility of it being due to anything else, nor to use really quite emotive language around it, when OP is obviously quite worried. It could be there is a good explanation. But it is also possible there could be something less benign evolving here- if we are basing diagnosis/reasons based on personal experiences, here’s mine:

One of my brother’s girlfriends (at a similar age) could be described quite similarly to this young woman. She acknowledged some people, whilst totally ignoring others-or gave one word answers if she had to-. She would turn her back on us without answering or even whilst you were talking- just walk away as though you weren’t there or hadn’t said anything. She did not appear happy if my brother was talking to anyone else- even his own family, in his own home, if she was there. She had him up to his room as soon as possible, where they’d stay until she left- other than to come out for refreshments (or rather, he was sent out to take to the room- my mother drew the line at meals taken to his room for the most part). She definitely did not want to spend time with his family/friends and gradually restricted what he did with them (and us) more and more. Initially, we were told she was “shy”, so didn’t speak much to people she didn’t know and didn’t want to be around his friends either for the same reason. She had friends, was in education and appeared to be able to interact at school/with her own friends etc.

unfortunately, it turned out that she was actually very controlling towards him and highly manipulative, negatively impacted his friendships, tried to isolate him from everyone and, ultimately, caused quite a lot of damage to his self-esteem. He was totally infatuated, or in love (depending in how you see it)- they definitely had quite an intense relationship- I recognise this is not uncommon in teens, especially first sexual relationships. He was quite vulnerable at the time, though, and it was a horrible, damaging situation for him. It was not a fun time for our family either. Ultimately, he eventually listened to friends and his siblings, saying “this is not ok for her to do this to you” and ended the relationship (which was difficult in itself as she didn’t take it well).

However, by your measure she would fit the criteria for selective mutism. And we should all have just been understanding, unquestioning and just accepted it as her having a difficulty that she couldn’t help- you know, just in case that was the case. If we had done that, I don’t know how long the situation would have continued.

I don’t doubt that my brother’s ex-gf had reasons for her behaviour, but it doesn’t excuse it. It may well be the case that this is in no way relevant to OP’s son’s GF, perhaps she does have mutism or ASD. Or perhaps she is just shy and a bit rude. I don’t think I can say, really. Nor can you.

It's really not that difficult for an autistic person to spot autism in another person. We're usually pretty obsessed with it and know as much as most experts.

I have an autism diagnosis as well.
The girl might do, but her description of her looking mardy as I said sounds a lot like my SIL, who doesn't have an autism diagnosis, and to my mind is more personality-disordered if I had to attempt to class her as anything.

It's really not that difficult for an autistic person to spot autism in another person. We're usually pretty obsessed with it and know as much as most experts.

They're very young OP so it's highly unlikely she'll be a permanent fixture. So my advice is to get on with your lives as normal and let this be. Even if she were the friendliest girl in the world, you'd still be seeing less of DS as they'd still prefer to be in his room, at her house or with friends than spending time with the rest if you, so try not to see her behaviour as the root of that. Carry on greeting her and speaking as you usually would. Don't make a special effort, and wait it out. If they're still together in 6 months then maybe a strategy is in order.

I think the op would be better to focus on her own family rather than navel gaze about the cause of this girl’s behaviour.

It sounds like her son is potentially vulnerable himself. I would be very concerned she acts this way around his friends and I’d worry about him becoming isolated as a result.

To the person saying that she's deliberately trying to ruin the atmosphere, you have absolutely no idea...an autistic person just isn't capable of this sort of manipulation. The reason they're so shy and awkward is because they don't have any social skills and they don't know what to say. They do usually develop, but very slowly.

The OP hasn't said that the gf has any sort of diagnosis though I don't think.
She's certainly been diagnosed within an inch of her life on this thread though.

I was like this too. I can remember one of my boyfriends saying that his friends called me 'The mute' and I had no idea what he was talking about. I remember being forced by my mother to do this awful pre-nursing course where we did different work placements at a hospital (which was completely traumatising). I overheard one of the nurses complaining about me, saying that I would just stand there staring. I was completely mortified and never said anything to anyone about it. But then there was another side of me that was very vain and I wore a lot of makeup and short skirts. I did often feel quite angry with my BF's dad because he was openly racist, but I don't think I showed it. I was easily offended by quite trivial things though.

I met exH when we were 18 and I don't think I looked angry - probably just terrified - and I probably said hello but it wouldn't have occurred to me to ask how his parents were. My BF would make me pop my head round the living room door and say goodbye and I found it really difficult. Remember her emotional maturity will be that of someone much younger.

To the person saying that she's deliberately trying to ruin the atmosphere, you have absolutely no idea...an autistic person just isn't capable of this sort of manipulation. The reason they're so shy and awkward is because they don't have any social skills and they don't know what to say. They do usually develop, but very slowly.

She may not have that level of awareness of her own needs yet.

Or maybe she just doesn’t feel like talking to the OP? Nobody is entitled to a person’s words.

If you're a guest in someone's home they are absolutely entitled to at least a greeting when arriving and leaving, and expressions of thanks.

I get that it is very hard for some people, for whatever reason, but equally I think that far too many young people now have been brought up without a foundation of basic manners.

Everyone on here a child psychologist , it seems.
On the pet and health boards , Mumsnet WARNS specifically that people may not be qualified and to seek professional advice from someone who has seen your animal or child.

Things can sound like XYZ yet be different.

Many conditions have similar symptoms.

The desperate rush to excuse “ by diagnosis “poor behaviours on here is crazy-

Unless it’s an adult male, then the advice is “ LTB” If he’s a sullen sulky asshole.

You don't even know if she's got selective mutism, that's just your opinion. You can't shoot down everyone else's opinion and thoughts on this thread. We've all read your opinion several times, you need to give others the same courtesy.

I insisted that my now DH said hello and good bye to my parents when we were first dating at 16 - he would quite easily have walked out without doing so. PIL thought I had great manners - well yes, because I was brought up to say hello, goodbye, please and thank you…

If theGF is laughing etc in the DS’s room but can’t say a word to his friendly parents, I would not be happy about that.

I fear I was probably like this at her age. I was raised by adults that could not be trusted, talking with them was dangerous. I wasn't taught and never saw my parents interact with others in a polite manner. I still look at my SIL and DH in wonder as they efficiently engage in polite chit-chat. Even as teenagers, such confidence!Honestly it's a learned skill.
I think my face probably looked angry all the time but really I would've just been desperate to get out of the situation.
Even today in my forties polite conversation is hard for me because my brain is telling me to run away.
I'm so glad my PIL were patient with me back when I met DH, I'm sure I looked rude and weird.

It’s reasonable to expect a hello, thank you and goodbye as a minimum and it’s important that your DS spends time with his friends without his gf. You are right to be concerned about this relationship, OP.

Yes I said that. It's very hard to get a diagnosis. That is not the same as it's hard to diagnose!

Sorry @SMiRAliser I've just read further back and see what you mean, I really should have the thread the correct way round

Is it? There's no services in my county who will diagnose it. Unless you can afford an independent salt assessment it will likely go unrecognised and undiagnosed by most

DH's brother is married to someone like this....they even have children together.

Watch out! It does happen🤣

Do people with selective mutism feel comfortable telling people about their condition to stop people mistakenly thinking that they are just being rude? If OP's DH's girlfriend does have SM, would she feel able to tell her boyfriend? I presume that if she did have a diagnosis and OP's son told his mum, OP would be understanding.