Her parents die in a plane crash and she moves to Wales to live with her uncle. She is a Welsh royalty sent through time to be protected. Her friend is also a reincarnation of someone from the past. Her cousin dislikes her and isolates her at school. The boys and girls have separate playgrounds. Australian author?

Merryll of the Stones by Brian Caswell

Her parents die in a plane crash and she moves to Wales to live with her uncle. She is a Welsh royalty sent through time to be protected. Her friend is also a reincarnation of someone from the past. Her cousin dislikes her and isolates her at school. The boys and girls have separate playgrounds.