This sounds like a complicated situation. I'm sorry that one of the parents doesn't want the other children to interact, very sad. However, even if you want them to meet, how much control do you have over it and how much drama will it cause everyone if it is forced?
The greatest lesson I've learned in the world of nuclear families and separation is this: If you can't control it, let it go. Some day they'll all be older and make their own choices and voices heard. Until then, don't let an issue you can't control cause friction in your own home. You can control that :)
Best of luck!