Yuki and Mika are brothers and sisters. Mika is always worrying about Yuki, concerned that the women he sees could bring back his memories or hurt him somehow. Thus, when Shuichi came into the picture, Mika relentlessly told Shuichi to back off and warned Yuki. Yuki doesn't like having Mika around much, he knows Mika is always trying to persuade him to go home.

Tatsuha is Mika's little brother. Mika doesn't come into much contact with him XD they don't talk at all in the anime. Mika doesn't worry as much about him as she does with Yuki or Tohma, and Tatsuha doesn't really have much for Mika to worry about anyhow. ^^ He seems independent and carefree enough.

Mika's father makes a few brief appearances throughout the show, when Yuki finally comes home. Mika's father has a temple and he's very religious and proper. Mika has great respect for his father, that's part of the reason why she wants Yuki to come home in the first place. His wife isn't mentioned in the anime, so there's nothing on Mika's or his relationship with her.