Sorry I didn't see past the first page when I posted.
I've found a brilliant organisation near me that organises trips out for SEN children. Sometimes just to share a meal or other times to fun things like the aquarium or today, they're going to the cinema (sen friendly showing). It allows children to get together an share social time and also gives me some time to myself while I know DS is safe.
Perhaps there might be something similar near you? They cater for children right up into their teens.
I'd also speak to your sibling(s) about their children. I'd be surprised if they don't have children with additional needs at their school and so should have an understanding. Could you also have a "our house, our rules" approach so that in your house, where dd can feel safe, she can always go first and play games her way?
There are lots of books to help children to have a better understanding of their additional needs, could that help to teach the cousins?
I really feel for you, it's so sad that even your family aren't helping you daughter to be included when they should be doing so much without being asked or prompted, they should just be there, doing it.
I do know that some people don't know what to do and so rather than trying and getting it wrong, just avoid doing anything - perhaps this is the case with your family?