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I’m looking for a YA sci fi book, maybe from the 70s or 80s, about a girl who lives in an orphanage/juvy work camp type place. She has the ability to move thru (time/space/another dimension?). There is a scary, mysterious maintenance man that works at the orphanage who turns out to be a boy shares her same ability, but tried it with his pet dog and somehow they merged beings. She befriends him and tries to help him and ultimately is able to un-merge them, and they have an romantic attraction. There is also an old lady that lives in a mansion near the orphanage who all the orphans go to meet. In the end it turns out the old lady is actually the girl’s grandmother who has been looking for her for years.