I hate the Velaryons so much in this game

I have been keeping the duchy of Dragonstone to myself ever since I noticed that every time I gave it to my heir, the fucking Velaryons would press their claim and kill them. I downloaded the interactive vassals mod to be able to defend them, but they just killed my pregnant heir faster than I could gather my forces to mess them up, and I'm so annoyed. I can't even wage war against them in vengeance. How do I take the claims to Dragonstone from them? It keeps breaking the immersion.