Hi, i think i can't help you much, but i know an ex boyfreind of mien is a tretow. Dennis Tretow. He came from Lübeck, now he lives in Leipzig. He also has a sister and a brother. His faterh is already death, so not much what i know. But i can tell you that ther are more tretow living here in germany.
So maybe trie facebook or so and try to write some tretow if they maybe know something.