Definitely a long shot, but I remember reading this book in the late '80s or early '90s.
It was about a teenage girl and her younger, early-teens brother, and they somehow got transported to another realm. In this realm they have to complete a quest, and maybe she has magic powers now (?). I seem to remember the brother picking up ground fighting and/or sword skills as this quest unfolds.
And the only part I can remember is how it suddenly occurs to her (much later in the book) how much her brother has changed, that he's grown taller and his jeans and shoes don't fit anymore, and he's shed his babyfat and has basically sprouted into a young man during this quest.
It may have been published in the 1980s. I would give my kingdom to find this book again. LOL Thank you!