妹快楽堕ち～お兄ちゃんに無理やりハメられてから～

One day, a big brother who blindly loves his little sister
eavesdrops what she talks on the phone.

She talks about future plans with a man and he hears that
they say something like "If we go to the same school...".

When it is implied that she has a boyfriend, he thinks in such a way, in despair, that the boyfriend must have bad influences over her so he confines her in the house in an attempt to isolate her from the guy.