Hmm... so if the question is why did they go back eight years and not some other time then it's possible this isn't about them. It's about someone else.
If it really is about them then this question doesn't add anything new.
If it's about brother-in-law and sister-in-law's redo chance then Da-ae and Han-eol are messing things up. But the in-law's seem oblivious so that's probably not the case.
But if it isn't about any of them... This is the time when brother-in-law appeared in sister-in-law's life. This could be a redo chance for the stalker and sister-in-law. That nails blondie as the stalker. It all depends on a reveal of his inner viewpoint now. We just got the girl's inner thoughts and, while she's upset about something, she's not acting like stalker-to-lover material... ... and Da-ae and Han-eol are indeed messing things up if this is true.