Responses
Hm, I don't think they'll lose their sister brother relationship unless the brother really manages to lose the mc's goodwill, and thus any chances that he'd try to intercede with the sis, in his behalf. Otherwise, the mc would surely try to make them reconcile, and the sis wold end up giving in out of consideration for the mc.
I feel so sad for the girl. To find out your crush is sleeping with you brother. I hope they don't loss the sister brother relationship.