Anti social SIL avoids interacting when over

I'm a bit like this - I'm autistic and really struggle at PILs, even though I like them a lot.

I wonder if you can actually give them the choice? Start again afresh

“it’s clear that SIL Isn’t comfortable staying with us for multiple days. Would you like to stop this arrangement? Shall we just meet every so often for lunch instead rather than multiple day things?”

open up the conversation. He doesn’t seem to have been given the opportunity to say this isnt working for me and I don’t want to be here

My DH has social anxiety and he would struggle massively even staying with his own parents. He wouldn't cope at all with spending the best part of a few days with anyone really. Even at home there is times he needs to be alone for a while to regroup

I can relate to this.
I am very introverted and am also autistic. I hate being a guest in other people's homes as you have to stay present and it is seen as rude to disappear. I have to disappear and recharge and to also try and prevent overwhelm. Being overwhelmed means I find it even harder to mask and can lead to me not being able to cope at all.
It sounds horrible but my boyfriend does not have living parents so this is not something I have to deal with anymore but I have had to in the past.
In the past I found it very hard. I even need time out when I visit my own parents.

Sounds like he is visiting for your DD's sake and you get to spend the time with her. He's not your other SIL.

My husband is a bit like this and suspected adhd asd. It’s his t coping method. He will find any reason to leave the house to go to the supermarket. I don’t like it but I’ve come to accept it

People come from different backgrounds and have different personalities. There are also lots of people with various undiagnosed conditions/traits.

It doesn't make this SIL worse and the other better. Being married into your family doesn't mean he will or has to be like the rest of you.

Accept him for how is as long he isn't rude and disrespectful.

Have you asked your DD if he's comfortable around you, or if you've offended, or if there's something you could do to make him feel more at home?

I mean - i'm sure you haven't offended and there's prob not much more you can do or say, but it would be a nice way to start the convo and see what she says out if interest.

From a different perspective, I would dislike to stay with my in laws at all. Not because they are bad people. But because I don't enjoy being a house guest for extended periods.

Going out for a meal or spending an afternoon with tea and good conversation are all lovely. But I would hate to stay over - I'd run out of conversation between breakfast and lunch. I'd also struggle to properly relax.

Similarly, when they come to visit us (they do stay with us a few times a year), DH's step father generally will take himself off to see a museum or activity near us. We will meet back up for lunch or dinner or something. DH's mum will sometimes just take herself off to the snug with a book. It's all good!

So, I wouldn't be too hard on him. He's probably a bit more introverted than you and your other SIL - it's not wrong, just different IMO.

It might not be long before they decide to just stay at a hotel nearby - there came a point when we decided it was best for us.

Be aware that he may have come from a very different background to you and as such, being 'at home' means something different to him. I was abused as a kid, growing up in a nasty unpleasant atmosphere and when with the family members, I was often being ridiculed, crying, being hit etc, so to slope off upstairs when told to 'get out of my SIGHT!' became a massive relief and a way of life. Unfortunately in adult hood, that way of life has not served me because as you say, other people find it odd. So I am once again the odd one out, through no fault of my own. What worked was to gently get used to people and to stop feeling the need to flee. I wonder if something like this is going on for your son in law? If so, give him time and enjoy the solo tine with your daughter and family, while he slowly unwinds upstairs and realises that nothing bad is going to happen to him if he sits with the people for a bit.

Fair enough I think. Spending time around family you're not actually related to can be draining.

What does your DD say about his behaviour?