According to Tenchi 101, Achika is Youshou's daughter, and Tenchi's father
looks and sounds like him because he is also descended from Youshou along a
different line.  (Youshou left quite a few descendents).  This means that
Achika is distantly related to her husband, but consider that Aeka was going
to marry Youshou and he's her _brother_....