Actually, Edgeworth never says he considers her to be his sister. If you think about how Edgeworth came to be part of that "family," and also take into consideration how Franziska's view of the universe is not always made of the Earth Logic, we really don't know if he would return those feelings. Calling them siblings is an interpretation of canon. It's a interpretation with canon backing, and frankly one I agree with. As siblings is how I generally write them. But it's not canon. It's an interpretation. There is a difference.