Tennyo Masaki is the sister of Tenchi Masaki. Her mother is Kiyone Masaki. Her father may be Nobuyuki Misaki, but he may actually be Tennyo's step-father.
Tennyo is an almost exact twin of her mother. She and her mother used to switch places when Tenchi was young.
Tennyo is much older than she looks. She is currently in her 80s. It is not explained if Nobuyuki is a normal earthling or is Jurian in the OVA series; this is why he may not be Tennyo's father.