L.B.
4 is a hard age, they want to be included in everything... and the older kids don't want to include them in anything.... Older kids think it is fun to play with littler kids.... I don't get it either ( I have a 4 year old and a 2 year old and the 4 year old tends to get shunned more then the two year old.
I stopped/limited the visits for awhile with my sister/sil for awhile because of the way kid were being treated, it seemed to do the trick. When they asked if they could get together to play I would explain yes, but you have to include everyone.... Explain that you understand they are bigger and that you understand they want to play by themselves once in awhile, but when kids are together they need to play together.
My husband tend to think I am over reacting also... But they are my family members that we live by and so I have to be careful what I say because they aren't his most favorite sibling anyway