Wow what a trash girl. She already said about having a girlfriend, but still can't give up? It's basically saying "how about two timing/cheating with me?".
Ah yeah, He still didn't deserve her even know about his past. Honestly I hope MC can get better fiance or go single, but since single woman in middle age it's filled to hardship better searched good man for her.
So that's why she so obsessed with her sister. Not just because her sister is the only one who cared for her. If you look from current timeline, you can somehow see that she will definitely returned several times.