In the world of this book, every time something happens, like a war, both outcomes come true. A new world is created with the alternate outcome. So there are many earths. The one that this one takes place on is not ours. In this Earth, America is called Atlantis.
The children survive the shipwreck because the boy is a witch and the sister hangs onto him. But you don’t find that out till the end. You think – because she says so – that she is the witch and she saved him.
He is left-handed, which where they came from is punished but where they have arrived at, is celebrated as it is a sign of being a witch. That’s all I remember….
I probably read it in the late seventies, possibly early 80’s. I was probably around 12 – it was a middle-year chapter book. It was really good.
I hope it sounds familiar to someone! Thanks for your help!