About 10-15 years ago, I read a saga of books, mostly young adult fantasy I think, about a girl discovering that she can use magic. She can travel to a parallel world where magic exists more commonly (but there is more like a medieval style), but is limited (because of an accident or similar) for everyone except for her.
On the first book they tell her that she has to find her lost brother, because he is the chosen one that will free the limited magic. However at the end she discovers that her brother can barely do magic and she’s the chosen one. I don’t remember much from the other books, other than the metaphorical “wall” that limits magic is described as a wall of thorns.
Also, she has a boy friend who has a lot of brothers. They both end up in a river somehow when traveling from one world to the other.