T.F.
My daughter decided to sever ties with her biological father because of the way he treated her, and instead of trying to fix it, he decided he would not send her anything, not even and empty birthday card.
Some people are selfish and spiteful. They do unto others as they have been done.
Try talking to them, but dont try to change them. If they want a relationship they will have one. BUT i can almost guarantee their son went and told them how awful you are. Now they are taking his side. Dont stress about it, do the right thing, thats all you can do. :)