@Spinaple "Wait didn't that other girl put the photos in her bag? Hmmmmmmmm"
I'm wondering if there is another setup, she (the photo stalker) hates her (sister of the main female heroine) and working with/using the other guy (creepy sempai) to discredit her by false accusation? If they destroy her (sister) reputation, it would allow photo stalker to get her objective and if creepy sempai now wants to assault sister then any defense she puts up would be suspect to any unknowing party. Now the question is why does photo stalker seems to have it in for sister?
One thing many of us mentioned in the past about different people (namely many of was suspecting MC's "girlfriend") in this manwha is that there may be more than two people who knows about the time shifting to the past, what if photo stalker is one of them? Many women are attracted to their friend's boyfriend/husband, what if she had a thing for the MC's brother, time shifted, and wants to remove the competition? Photo stalker would know about about MF (main female heroine), sister (friend in the past, but with second chance competitor), brother, and MC so she would know who to look out for (she might know enough about their past to realize who remembers the past) because the MF keeps popping up around her sister in an almost intentional way and realizes that some kind of interference changed some of the past of sister (photo stalker, if they were friends, most likely knows about how sister and brother met for the first time). Just a theory.