>>281209428 (OP)Because bro-sis relationship is borderline sexual?
All teasing and fights between brothers and little sisters happen because at certain age, they realize they shouldn't be so close to each other, because society thinks it's weird.
So, the only way to get attention is to pick on each other. But they do this out of love.
It's the same thing between male friends. All the banter, trolling and pretend-fights is our way to get close and physical without looking gay.