The brother isn't malicious. He really just wants to be friends/maybe more? with her because of how she helped him back in the day. The girl who her ex cheated on her with, is a classmate of theirs who was obsessed with older bro and didn't like our main girl. Big bro tolerates her but doesn't know the connection or something. Something happens, big bro backs off and our girl and issei get closer. This was someone's translations of future chapters but they said their Japanese isn't great so who knows. I could be totally wrong.
The brother isn't malicious. He really just wants to be friends/maybe more? with her because of how she helped him back in the day. The girl who her ex cheated on her with, is a classmate of theirs who was obsessed with older bro and didn't like our main girl. Big bro tolerates her but doesn't know the connection or something. Something happens, big bro backs off and our girl and issei get closer. This was someone's translations of future chapters but they said their Japanese isn't great so who knows. I could be totally wrong.