Caught brother and sil laughing at my dp behind back

But again, he is who he is. It's not the other people's fault that the ice hasn't broken, if he's reserved and quiet in company. It probably made it funnier for your brother and SIL if this man who's always been quiet and reserved suddenly starts sprinting across the car park after a 3 hour drive to do a non-time-sensitive activity. (Or was he dashing off because he didn't want to have to talk?)

This.

I think this is really hard to call, it can be natural to laugh out of surprise or at the absurdity of a situation without there being any attempt to be ridiculing anyone.

The fact that your partner was so far ahead that he couldn't really be included in anything Bro and SIL said and the fact they were trying not to set each other off makes them seem more sympathetic to me. But it's really hard to know without knowing all the people involved and without being there.

I wouldn't say anything but keep an eye on future interactions to see if anything else happens.

I think you're actually projecting your own feelings about your partner's 'odd'/'rigid' behaviour onto your brother and SIL. You admit yourself that it wasn't time-sensitive, and that you said to your DP to 'take in the moment and chill' before you even saw their amused response to him darting off like a racehorse, so you registered yourself that his behaviour was out of the ordinary for the scenario. They don't know he's considering seeking a diagnosis for autism, so they are purely seeing him rushing off as soon as the car drew to a halt as odd, rather childlike behaviour. And regardless of whether he gets a formal diagnosis, those behaviours will still be out there for people to see.

I think you should be thinking about your own feelings about his oddities.

It sounds funny/cute/charming to me to see a grown up run towards a planned activity. Sorta a Chevy Chase National Lampoon Vacation movies scene. Much prefer that over people who want to leave early or bitching about parking or costs. Since they were behind you and dp I don’t think it’s was malicious or judgmental. Just a laugh. I love a good laugh. And laughing at ourselves is a very important trait so I would let it go. Dp didn’t see the giggles the rest of the day went well and it seems you all enjoyed yourselves. I’m always vigilant to people being “made fun of” irregardless of age diagnosis etc and you can advocate for DP but I think in this instance you also could have called back “I see you two laughing back there DP is sooooo excited!!”

He was too far away.

I think its worse that they thought it best to stifle it rather than include your DP in it. Would have been better to have said 'steady on mate, we've not even got our boots on yet...' or whatever, then it would have been a shared joke rather than laughing behind his back. If it happens again I'd do my best to open it up and pull DP into the humour. Because yes, sometimes people's behaviour is funny, but you laugh with them not at them.

It's so awful when you get inappropriate giggles. I'm not saying they were right Op, grown ups should be able to control themselves. But I am still cringing and kicking the sheets in the night over an incident where my MIL and I had to leave a Good Friday church service because we simultaneously saw the expression of extreme piety on the priest's face.....

Is the activity important , for example were you going to the beach, parked up and he went running off to get to thr beach, leaving you all behind, and to get the stuff out the car, with you chasing after him shouting for him to chill?

I did it was more odd and rude and my expression would be more WTF rather than laughing. He behaved like a child who just ran off. Surely you can't be upset when you know something like this is odd.

You’re being incredibly over-sensitive about this.

Sometimes, people’s quirks are funny. It’s fine to acknowledge that. They weren’t sneering at him or taking the piss. They were laughing at something that was funny/quirky. In my family - and I think most families - laughing at something like that would mean that we found it endearing.

I’m dyspraxic. Last time I saw my family, I was getting in a right old pickle putting my coat on and it made my mum and my brother laugh. That then made me laugh too. It was an affectionate laugh at something that is objectively funny and also quintessentially ‘me’ in their eyes. It was OK for them to laugh because there was no negative judgement involved.

It is odd and funny. You could tell your bro and sil that you're aware he has some odd traits but he's wonderful and kind etc. and that you both suspect he's on the autism spectrum.
Being open about it will change the dynamics to a more positive one.

I'd probably shout 'run Forrest run' after him.

Weren't you also rolling your eyes when he darted off? In a gentle 'What's he like?!' way.

They had the reaction that most people would in that situation, but sometimes it gets amplified if you're laughing along with someone else. They weren't taking the piss, just sharing a giggle over something quite amusing.

But it was not deliberate. I mean they pulled up and he ran away, leaving them behind as he was so excited,I’d probably have laughed as well,at the sight of it and the op chasing after him.

I think they shouldn't have allowed themselves the fun of mocking him behind his back where there was a good chance you would see them. It is insulting when someone does this in a way that suggests you are too oblivious or stupid to notice a fairly open behaviour.

However I think most of your negative emotion comes from being confronted with a really obvious incidence of his odd social behaviour (and his total lack of response to your efforts to bring him back on track) combined with their normal but mildly unkind reaction. This sets out clearly what you are taking on by being with him, despite his many good qualities and the fact that some of his quirks actually work really well for you.

I'd think it was very odd and rather funny if having arrived at our destination a grown man just ran away.

I'm with you OP, I think their behaviour was unkind.

I also think there's a difference between laughing at your own parents or siblings, and laughing at someone's partner.

surely its more hurtful to do it to someone's face?

OP's DP hasn't actually been diagnosed with anything, and there's nothing to say that her sister/BIL have even been told that's something they are thinking about.

Therefore to them, it's just a funny personality quirk/mildly annoying habt.

Teasing one another about personality quirks or occasionally getting a bit annoyed by them is completely normal in any close family or friend group - do none of you really EVER have a giggle or roll your eyes at the messy brother, or the chronically late mother, or the father who says he's ready to go then sits on the toilet for half an hour, or the grandad who forgets to put his hearing aids on, or the new mum fussing over the PFB, or the friend who is a VEGAN and makes sure everybody knows it, etc....?

It doesn't' mean they don't like him, just that everybody is a bit annoying sometimes, and if you spend a lot of time with people occasionally they will do something that you find annoying/funny and vice versa.

I can't really see why it was so hysterically funny tbh. But would he have been ok if they'd ribbed him about it? Would he have seen the funny side? I think it would have been better if they'd involved him in the joke rather than sniggering behind his back. They could have called out 'Give us a chance!' or something. It's tricky to know without knowing the family dynamic though I think.