I read this fantasy book in late 80’s/early 90’s as a child. It featured a brother and sister. At least one had magic (maybe both). I believe brother had a magic scepter. Possibly had a large bird they rode to get to places. The cover may have shown the brother with the scepter riding the bird, but I may just be making that up. Good luck and thank you!
Could this have been Steel Magic by Andre Norton?