What I remember: I *believe* it was about two suburban or city-dwelling high-school-age sisters, one or both of whom had sort of magical powers, though the world was not overtly magical. They had a grandmother or something who was magical, maybe?
One of the sisters was a really great dancer, probably ballet, even though it is specifically mentioned, probably by her dance teacher, that her body is all wrong for it--the implication being that her dance ability is some kind of magical gift.
The other sister has the power of telekinesis, and at one point gives one of her teachers "dropsy" by making her drop things in class. She or the other students may make fun of this teacher by using the word "dropsy," not sure.
The only other thing I remember is that the words "love child" are used in reference to one of the sisters. I remember this, because at the time I was like, "What the heck is a love child?" (Which may also be an argument for the book being an older one.) I *think* it was said that the sister was a "love child" because she was illegitimate, but also because she was loved or came from love or something...
Help? Am I doomed to vaguely remember this book forever, but not remember it enough to actually ever find it?