Just that really.
He used to be my closest sibling (not in age), we spoke regularly, he once make the long trip to stay with dh and I for two weeks.
Since he got together with his partner I just don't hear from him at all anymore.
He's not bothered with any of my, dh or dd's birthdays. I can be a big girl about me and dh but dd absolutely adores him and has been really hurt with not even a card or message on her birthdays or Christmas. Before we'd always send presents and Skype etc. Not even an acknowledgement now.
He's had other long term relationships before so I don't think it's the classic thing of guy gets girlfriend type thing.
His wife has always been REALLY frosty with me. I'd met her three times before the wedding and each time she made no effort to talk to us at all. At the wedding she was actually pretty rude and dismissive every time we tried to engage her. Her family were exactly the same.
He's also dropped my parents to some extent. They both aren't in the best of health, he's gone from seeing them a few times a week to calling maybe once every couple a months and only seeing them every 6 months. They live close by to him.
He spends every weekend with his wife's family. He's become a Godparent to his wife's nephew, they all holiday together.
I know people move on when they get older, but I'm really mourning not having him in my life anymore. We've had a couple of very big life events happen recently and not heard a thing from him.
Nothing bad has ever happened with us as a family, we're not toxic or have caused any reason to go low contact, I think he just really prefers her family. 
It's possible that he find our side a little embarrassing. My parents are working class whereas hers are very well to do. He's told my parents how to behave around them a few times. Not that my parents have anything but lovely manners, they just have quite strong regional accents and obviously don't have a lot of money. Her family do fun swanky holidays five plus times a year. Ours are more - bbq in the garden and go for a nice long local walk type.
Has anyone experienced this then had the family member start up more contact again as time goes on?
I'm at a loss of what I can do (nothing obviously). I continue to send cards, presents etc at birthdays, Christmas but now don't even get an acknowledgement of receiving them.
It's horrible, he's gone from my life and I really miss him.