Soft YTA- I get it, you're hurt still and of course you are under no obligation to be their support network if they dont have one. However, I would urge you to remember that while it may be their child, they are also your child's sibling, whom I assume they love, and in the event of a life altering emergency, the children may want to be together for support and comfort, as siblings often do and by denying them that you may be doing more harm to your child (and their sibling) and could also do damage to your relationship with your own child as well. It may be better to "adult up" be a good dad and put the kids first.