OP you sound like everything is well intentioned but I agree with PP you need to back off.
I was extremely close to my brother. He came everywhere when we went out as a family.
He met sil, they had kids and I see him about twice a year now. Once over Christmas. I don't get on with SIL, she is controlling and has made clear her family come first.
When dbro met her I was very careful to not overstep boundaries. I used to organise all our parents presents at Christmas and birthdays but when he met sil I asked him what he wanted to do. I didn't assume they would still want me to organise something.
When their baby was born sil told us we weren't to come round for the first two weeks. It hurt but I said that was fine, whatever she wanted was good with us.
Not everyone wants a fuss or family round a lot. Some people want space and distance.
The more you push it, the more they will pull away.
I know you are excited to have a new nephew, but the child is their son. It may not be the way you expected it to be. But you should accept it is what it is.
Mum always thought her house would be full of grandkids at Christmas. With her kids and their partners.
It's not like that at all. She spends one year with us and one year with them. Every year she spends with them they try and cancel right up to last minute.
But she accepts it is what it is. That way she isn't say over analysing they why and what ifs.