I feel like you might be why they keep the distance! When they get older they can. Dont push your kids on someone… They will get hurt.
No… let it go. It’s a half sibling and when they get older they can choose to find each other. Doesn’t matter if they’re related … that’s the current wife or girlfriends child and you have no say in what she does with her child. If it was reversed you wouldn’t want her having the power to get involved with your kids. It’s the fathers responsibility to be seeing his children and having them in each other’s lives, if he’s not even in your kids lives why would you even care?? That’s crazy to me. You should be more concerned how he doesn’t see his children with you. 
If he’s a deadbeat then even more reason to forget all about him and his family.
they do deserve it but not much you can do if the kid’s parents are willing to establish the connection. wait til your kid is older and let them be aware of the sibling growing up. they can find each other on social media