The brother in law gets found out by everyone and he commits suicide for the Mana's happiness and his own sadness. Apparently, from what i read from another forum, he got the female protagonist pregnant and she wants to commit suicide as well. However, her boyfriend stops her. They eventually get married and she has a daughter that looks like her.
SPOILER (if you want to know the ending)
The brother in law gets found out by everyone and he commits suicide for the Mana's happiness and his own sadness. Apparently, from what i read from another forum, he got the female protagonist pregnant and she wants to commit suicide as well. However, her boyfriend stops her. They eventually get married and she has a daughter that looks like her.