Tensai Family Company(1994)

Tensai Family Company

Katsuyuki, a gifted, petty and egocentric high schooler, intends to work in the high financial world. His well regulated life explodes when his mother remarries his worst nightmare: a traveller writer, unproductive, nice and completely unpredictable and father of a son who looks like him. All his opposite. Consequently begins the subtle contamination of his entourage by their terrible joie de vivre. Even Nagasawa "the female brain of the high school", the only one for which he had respect for decided to find anew his peace. Katsuyuki feverishly seeks a way for returning these aliens on their planet and he's not sparing with spite.

(Source: Shoujo Manga Maniac)