Been so long I had to skim the previous 3 chapters to see what was happening.
Bro got both of them to agree to all 3 of them to sleep together by doing nothing, I wonder what he'll have to do to get a 3some out of them (probably still nothing, do nothing win stays unbeaten).
I think it's MC. His passiveness (which is understandable since he was even younger when they dated) probably made her to be the one chasing. And to a family member, it might seem like she was being used.
Since the sister doesn't know the ex, she might have assumed that she was dating a useless adult.