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"Escape to Witch Mountain" was written by Alexander H. Key and published in 1968.
It's about alien twins Tony and Tia and the Blue Ridge Mountains of North Carolina.

https://www.goodreads.com/book/show/458191.Escape_to_Witch_Mountain
>Then Tony and Tia find that they must escape from men who want to use their special powers for evil.
>They begin a desperate search to find their true home--which leads them to the strange and mysterious Witch Mountain.

Witch Mountain is Brown Mountain.

https://blogs.lib.unc.edu/ncm/2009/03/13/race-to-witch-or-brown-mountain/
>New in theaters today is a movie with a strong North Carolina connection: Race to Witch Mountain.
>the book was set in the Blue Ridge Mountains of North Carolina and Witch Mountain was based on our state’s Brown Mountain.

The twins are helped by a Roman Catholic priest.
They are Castaways.