We don't know yet if the sister and the ML had THAT sort of a relationship. The engagement was "arranged" after all. Furthermore, the ML didn't seem to care that much the sister broke up the engagement to be with another man. He was more focused on finding another bride to replace her. Since he only wanted the engagement due to the "family custom" that dictated he needed a bride from that family.
It wouldn't surprise me if the sister only entered the other relationship because she got frustrated that the ML wouldn't "warm" to her like a normal fiancee.
It seems based on the parents' treatment of the FL the sister is used to being the focus of attention and "loved" by everyone while the FL gets discarded in comparison.
Notice her expression when the FL tells her that their relationship is going well and the ML even confessed to her. The sister looks shocked and even a bit sad.
no they didnt have an sort of intimate relationship. remember, she had a bf? she ran away with him
I tried to forget about it but it really is weird after fucking who her sister moving into her like there wasn't a big explanation on how their relationship was whether they never really loved each other or maybe even Sakura's sister had a one sided thing for him but eventually fell out of love and left for the other guy. The possibilities are endless