Wait so they were actually dated in HS?
Yes and broke up. They discussed this earlier in the series when Mii-chan gave our MC advice about our MC's new love. She basically told her not to make the same mistakes she made during their relationship. Seems like our MC lost interest in Mii-chan or simply moved on, so Mii-chan warned her about her tendency to lose interest in things, being too carefree.
That could be Mii-chan's version of their relationship though and our MC might have had other reasons for their breakup. We'll see once we learn what actually happened.
Because of their background, and how uninterested our MC seems in Mii-chan as a romantic partner, I think it's highly unlikely they get back together. Doing so would mean our MC once again lost interest in her current "contract sister" love and shifted to "something new" which would be Mii-chan. Can't imagine that would be the story.