Known (and presumed) members
The three sisters:
Takeru's mother, aunt of the Three Sisters (Indirectly mentioned)
Minami parents (heavily implied to be dead)
Though very different, the sisters share marked similarities, such as high intelligence (repressed in Kana's case), a hidden domineering "Banchou" side (explicit in Chiaki's case), a reserve of kindness (explicit in Haruka's case), a certain quirkiness, including occasional tendencies to reach seemingly logical conclusions that lack common sense (Minami Logic, very common in Kana's case), and a lack of experience in dealing with romantic affairs. Haruka claims the missing Minami father to be quite similar to Fujioka. It is likely that Takeru also draws certain of his traits (his affability and the manner in which he is easily flustered) from this similarity.
The three sisters are very tightly knit and loyally look after each other. Takeru visits on a frequent basis to keep tabs on them and it is implied that either him or his mother is the official guardian of the three, but this does not seem to be mentioned in the manga. It is not clear who finances their living arrangement. The sisters' given names follow a seasonal pattern, from oldest to youngest, namely from "haru"(春, spring) to "ka"(夏, summer) to "aki" (秋, autumn).