Haha! They can't take out coven after coven if they were not powerful. lol. But your right, they are going to both need protection which might only bring them closer together. Though her sister might need protection from him rather then the other way around. I do not see him going easy on her sister just because she is related. 

I'm still trying to think of something too. Surely between the two of us, we can figure out a power we like.