This has been going on a few years now. Looking back, they went a bit funny after having had kids and spent more time with dh brother, who had children the same age.
When we had our daughter, ;they weren't overly interested even though she got on well with the cousins.
Dh wasn't invited to his cousins 40th, aunts 70th (she claimed we hadn't returned her calls but she left no messages ) and dh did not get a card for his birthday (another milestone).
His brother, on the other hand, is in constant contact. We have never had a whisper as to why and no obvious falling out.. I saw his wife a couple of years ago à nd she was perfectly friendly and got a brief text when dad died . Dd is now on instagr am and the cousins have now all started following her and sharing photos and memories she has no part in. The girls are perfectly friendly so it's not them. But dd is bewildered by it all
Dh is very eaten up. To me it's neither here nor there but I feel v sorry for him and ddbut dh will not ask why which I would if it were me.