So this is a long shot, as I unfortunately don't remember many details, but I am trying to find a book I read about 20 years ago when I was a kid. I read it in my local library (in the UK) so it would have been late 90s or early 2000s that I read it, though the book could have been older.
It would have been from a female protagonist's POV, with a young girl (aged somewhere between 10-15) who somehow found she could access a magical realm, possibly through or with one of her family members. I think she had a brother and mother (I don't remember a father character but there may have been one without much focus on it). I think it centered more on family dynamics/relationships than friendships or any romantic kind of relationships.
Again, I don't remember much of the "laws" of the magic realm; people in the realm had magical powers and I assume the girl did, but I don't know if she only had powers when in the magical realm, nor do I remember the specifics of how she "travelled" there. I think time passed differently in the two worlds.
I think there was some sort of evil witch/sorceress who was targeting the girl as a threat, either because of some sort of prophecy or because the girl had such strong magic potential that she could grow up to be a threat.
It would either have been in the 5-10 age range or young adult. Likely a series, though I don't know that I ever got the chance to read any others beyond the first one.