TLDR douchebag brother steals his own blood related little bro's girlfriend who's a magical girl.
She breaks up with MC and gets pregnant of the brother, the baby is a girl. Brother still goes out to cheat on the heroine, she knows he cheats but tells that no matter how many girls he meets he only love her (MAXIMUM COPIUM).
Then comes the magical mascot and tells her there's a monster nearby so the MH transforms and goes leaving the baby girl Yuna in the hands of MC.
Meh, I say MC should fuck their daughter once she is of age.