Can you help me find a book?

I am looking for a book I read when I was younger. THis is what I can remember: There were human telepaths and the strongest were called primes. A boy and his sister were raised in family goups with aliens that I think were called murtogh or something like that. I remeber the boy goes onto a human ship and gives one of his companions cleaning water to drink by mistake and poofs himself all over the ship to get help. Also, I remember the Cpt. of the ship shook the boys hand and then apologised because that was too familiar with a prime and the boy said that was ok and thought to himself that the Cpt was like warm and brown I think. Any help would be apreciated. It has been driving me nuts.