Not to stop my son coming over when grandson is in bed.

I'm of the opinion that your son need to learn that other people exist and he needs to make allowance for their needs too. It feels like his autism is being used as an excuse for him to not have to think of other people's needs.

My dd recently moved out after her and her baby had lived with us a year. She certainly had no say in the comings and goings of her siblings visiting.

Entitled madam springs to mind here op. If get conditions aren't to her liking she can move out. .

Children get used to background noise

Should he also have to change his whole routine with his mum just because his sister has moved in and can’t settle her baby? The sister also has a responsibility here too.

It really does shine a big light on his attitude and how he was raised. That’s not autism - that’s just a selfish prick. I’ll bet my house that if he ever finds a life partner, they’ll end up leaving due to his selfish attitude as it won’t just be around this issue. That’s a sign of someone fundamentally selfish who was never raised to think of others - something a lot of boys have been let away with.
Look at the contrast with the daughter; being told by her mum to change her expectations and needs to allow the male child to do whatever he wants, without showing any consideration for anyone else.

Yes, he does sound disrespectful. He’s f he is managing to function, hold down a job, live independently and create a routine he can sure as well shut the fuck up and show some consideration

How about also looking at this line - I do keep saying shhh when his volume creeps up and he tries to be quiet but then he’ll forget and laugh or his voice will slowly raise again. It’s evident he is at least trying, not just blatantly disregarding everything.
As I said above, OP’s dd also needs to take some responsibility here - it seems like she’s expecting the whole house to go into shut down from 7pm just to keep the baby asleep. Thats disrespectful too. It shouldn’t take longer than an hour to settle a baby to sleep, and said baby should be able to sleep through general noise.

DD needs to be more relaxed about her baby’s bedtime? Maybe it’ll be easier now heatwave is over.

However my son doesn’t seem phased by this as he doesn’t feel it’s his responsibility to be quiet and not wake him up,

What a shame your son doesn't have respect for the people that actually live in the house he's visiting

You say he's autistic and can't help it which I understand as I have a friend like this, just has no awareness that he's shouting inside. But from the line above I quoted it sounds like he's not even trying to begin with. At least if he was trying he'd be being respectful while waking the little one up.

I presume your dd has asked several times for you and him to keep the noise down and has been met with :oh I cant ask ds that hes autistic" or she has heard that a lot in her life and has the snapped and said he can't come round.
Your dd cant ban people from your house but you and your ds need to be more considerate.

Both your ds and your dd need to be more considerate here. Your dd needs to get her ds to sleep more effectively - it really shouldn’t be taking more than an hour to settle a child. Your ds needs to try harder to keep quiet, albeit with frequent reminders. Unless he’s deliberately going out of his way to make as much noise as possible, it’s really unfair to expect him to creep around everywhere, the same as you shouldn’t be creeping to the loo either.

My son who has an ASD diagnosis had terrible trouble sleeping when he was little. It was partly caused by anxiety. Being woken up in the way you are describing would put him into a horrible cycle of falling asleep and then waking becoming more and more anxious. Having an uncle with ASD massively increases the chances of your grandson also having it. If you will not ask your son to quieten down for your daughter, would you do it for your tiny grandson?

100% agree

my friend has a severely autistic child and has worked extremely hard to help them manage situations so that they don’t allow the autism to get in the way of their life and integration with others. This has meant deliberately challenging the OCD type tendencies to wanting things in places, same foods on same days of the week, eye contact, and what they say and how and when they say it.

we’re raising adults here, adults that will have to function without us one day. It’s the least we can do.

”oh he’s autistic, not much I can do” is failing him. Sorry, but it is.

You keep bleating on about ‘oh he’s autistic’ like that’s an excuse. He clearly works and lives independently so he is able to function in society. He can therefore perfectly well understand shut the fuck up.
My 9 year old nephew is severely autistic and will likely never live independently and even he understands he needs to shut up when his baby sibling is in bed

If there’s a way for him to be quieter, then yes, you are being very unreasonable.

He manages to live under and work but can’t keep his voice down or not be “heavy handed” in the bathroom? He sounds AWFUL.

They're both being unreasonable. Your daughter can't dictate who you have in your house - especially when it's your own son - and will have to accept the circumstances of your home along with your help. Meanwhile, your son's attitude, "he doesn’t feel it’s his responsibility to be quiet and not wake him up" is unacceptable, autistic or not, and not a great way to conduct yourself in the world.

I think a bit of both, your grandson should be able to learn to sleep with a reasonable level of noise going on downstairs- tv, normal talking etc.
However sounds like your son is being inconsiderate and very loud, surely he can stop shouting? You say he’s autistic but also able to work so he can’t be shouting all day at work?
I would ask your DS to be quieter when he comes around to begin with and see how that works out

Asking him to not come isn't a reasonable option. He is autistic and somehow manages, but it seems that coming to see mum is a part of his support network allowing him to manage.

Is there any way to make the baby's space more insulatwd/white noised/something else to make it easier?
If the baby wakes up from someone hlgoing through the front door, it can't be helped.

Sorry but ANYONE who doesn’t live at an address coming to see someone who DOES literally IS visiting. Therefore, they are a visitor

anyone coming into the house regardless of relationship or title needs to be considerate of those who may need quiet

any single parent having put in a shift with a little one all day - especially with the recent weather- will be counting down the seconds until bedtime some days because they’re exhausted themselves

someone steaming in on a regular basis not making any attempt or adjustments IS an inconsiderate prick and does need to be told to keep noise to a minimum, until the little one is spark out and in deep sleep.

I know how hard volume is for autistic kids, I have one, however you need to remind him every time. I'd say he can come round, stays at opposite end of house form baby and keeps his voice down. If he can't is say he should reduce his visits, surely? For a short while?

I know that not every job requires you to be queit... I more so meant that he can function in that he lives independently, has a job, is capable of travelling to work and to the OPs house etc. So he knows how to conduct himself.

I also agreed earlier that the DD cannot request he doesn't come.

Lots of jobs don't require you to be quiet!

I think it's outrageous of DD to ask him not to come. I think it's reasonable to explain to him he really needs to try.

My son is autistic. This wouldn’t fly in my home - he’d be told to be considerate of other people and be quiet. But I’ve been teaching my son to manage his habits since childhood, it sounds like you just never bothered.

Your kid live independently, has a job, functions fine so he absolutely can learn to be quiet. Some people with autism could never learn, but your son can. You just haven’t bothered and you absolutely are playing favourites.

If you cannot be considerate of a baby sleeping and would rather cause your daughter extra hours of stress and sleep deprivation because you cannot be stricter with your son, then you are playing favourites.

He needs to shut up.

Sounds a lot like favouritism to me. You’re essentially telling your DD (and your GS) that they need to just deal with the disruption caused by your DS because you’re worried about him feeling unwelcome. How come you’re not worried about your DD feeling unwelcome? What if when your DD eventually moves out she chooses not to come round so often and you don’t get to see your GS much?