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

I think both of your children need to be a bit more respectful or the other.

Daughter could get get a white noise machine, or have a low story on to settle son to sleep, and understand that her brother comes to see you.

Son needs to be mindful of nephew sleeping and try to keep his noise down.

agree. OP is pandering to him

Your daughter is being very entitled. Up until now the arrangement worked for your son, and for you, who is the homeowner. She is very lucky you are accommodating her (I do hope she works/is looking for work and is helping you out rather than just freeloading).
By all means try and get him to keep the noise down/sit outside to chat etc. But the whole family should not be held to ransom by a kid's bedtime. I say that as someone who has a DH with a very loud voice.
And sorry but I attend weekly social meetings at a friend's house where we have dinner downstairs followed by a discussion related to our group - they have a baby upstairs. They have to suck it up, use a white noise machine, go and settle or just accept that this happens sometimes and life doesn't revolve around a kid's sleep.

I feel for your dd having had a non sleeping 1 yo myself. Having said that it is your home and it does sound like you try and encourage your ds to be quieter so I do think she is being unreasonable to ask that you don’t have your ds round. Is a bedroom swap possible so that the 1 yo is in a quieter room? Or I had sone success with a white noise machine (or app and speaker) that sort of helps screen out background sounds.

But her daughter and grandchild are also equally family members, and the sons noisy visits will be making the daughter feel unwelcome.

If he has the capacity to live independently and work, he has the capacity to keep his voice down when someone is sleeping. He's choosing to make his sister spend hours resettling the baby simply because he "doesn't feel it's his responsibility to be quite" thats an active decision to be cruel to a struggling mum (who's clearly having a rough time if she's had to move home), and a child who will be overtired from been distributed all the time. Sleep deprivation is literally a form of torture, so it's quite concerning that he is choosing to inflicted it on a child, and your supporting him to do so.

He is obviously functioning well though if he has a job? So it shouldn't be a hardship to remind him to keep it down?

Could you ask him to call you when he's outside and you let him in or he comes in the back door etc. I think your daughter is being reasonable, having a little one who is a difficult sleeper is a nightmare. She wants to get her daughter to sleep and relax. Could your daughter consider playing white noise or music in the same room as sleeping little one.
Your son is autistic, so is my daughter, she's improved some of her social skills and consideration towards other people. Maybe your son could too. Thinking about other people and adapting is a life skill.

It’s your daughter’s home too. If he did come in every night and wake the baby the problem be solved with your sleep deprived daughter murdering him in a homicidal rage. Can you not think back to when you had babies and how you’d have felt if someone had woken them up a few nights a week? They aren’t babies forever, I’d see if your son could meet you somewhere else once a week and only come around to yours once a week if he can’t not wake the baby, can you afford to shout him a drink at a cafe or pub? But while it’s summer you could just go for a walk.

But he’s independent no? Or does he require carers?

it sounds like you’re only concerned about upsetting your son and don’t really care how it’s impacting your daughter or grandson

@wildorangetruck what time does he finish work can he come earlier ? Or maybe you visit him some nights

God he sounds unbearable.

I wouldn’t be ok with telling son he can’t come round after work any more.

but give and take on both sides is probably possible here - presumably he works and so can’t come in the day.

if he is autistic then this adds complexities - is he in supported housing/does he need help with things in the evening?

I have a young adult dd with autism and I check in with her most days.

YANBU to not stop your son coming over, but why can’t he use inside voice volume, has he always been a selfish boy?

I agree with someone saying you seem to play favourites

your poor daughter, can only be back home for a difficult reason such as job loss, relationship breakdown, something like that and now she has to deal with golden child behaviour

you can enjoy your sons company at a reasonable volume

If your Autistic son is capable of working, he is capable of understanding that sometimes you need to be mindful of others. I can’t see anything wrong with saying he either keeps the noise down when he comes over at night, or he stays away until he can visit in the daytime.

I think everyone needs to compromise. Your daughter could investigate white noise or similar to distract her DC. Needing silence to sleep can make it harder as they get older anyway, but appreciate all babies are different.

And your son could be asked to just keep his voice down and be quieter when coming over. No need to stop but to be considerate?

I think she’s unreasonable to say he shouldn’t be allowed to come round.

But you and he are unreasonable to not think it’s even possible for him to be asked to keep it down. His sister is struggling with her young child - can’t he be considerate?

This is a tricky one, and I see from the very divided comments that people have strong opinions one way or the other. I think whatever you decide is going to impact someone but I’d err towards helping your DD if the situation is temporary (especially if she’s just left an abusive relationship). It’s going to be hard for her to get back on her feet with a little one and I’d prioritise giving her the best chance of sorting her life out by actually getting her evenings back so she can apply for jobs and househunt. You can phrase it to DS that it’s just temporary that he has to either keep his voice down or keep visits to weekends, and it wouldn’t harm him to learn that this is what it’s like with babies. Regardless of whose house it is the baby didn’t ask to live there.

I think your dd is lucky you can provide a home for her and can’t start setting conditions for living with you presumably at a huge saving to her and inconvenience to you.
my adult dd has had to come back and live here and I bloody hate it. I’d like shot of all of them tbh!
remind your ds to be quiet when you open the door until you are somewhere you can close a door. Get the white noise thing and your dd will have to manage

He's not toning it down because he isn't being asked to. Of course it's unreasonable for him not to come over but it's just common courtesy to be more aware that a baby is trying to sleep, especially if sleep is an issue.

When he comes over, just politely mention the baby is being put to bed so tone down the volume. Yes the baby will eventually get use to it but honestly it can be sould destroying spending ages getting a baby to sleep only for someone else to awaken them with no care!

Give and take on both sides.

Your DD needs to get a white noise machine.

You are being really unfair in your daughter! Yes it’s your house but at the moment it’s also your daughters home, due to whatever circumstance has forced her to your house.

tell your son to calm his big booming voice, and stop playing favourites

Well there needs to be some effort and compromise from both sides. I would speak to your son and ask him to be more mindful of his volume levels. It’s fair to ask him to be more thoughtful.
On the other hand your Dd is not really in a position to say he can’t come over when you and he wish him to. The baby sounds very sensitive to noise. Have you tried things to mitigate the noise he heard? White noise?