[/The yellow haired asshole(which the FMCs sister is dating right now) has also returned to past i think. And the accident which resulted in the death of the MCs and FMCs siblings is also the fault of the the yellow haired asshole. He is trying to get the FMCs sister as he couldn't in the past. That's why he and the girl (who was supposedly harrased by the creepy ugly dude) planned to falsely frame the creepy ugly dude and the stalking so that yellow haired asshole can get closer to the FMCs sister.
And also the reason why they were sent 8 years back i think is because so that they can permanently eliminate the threat of the yellow haired asshole ultimately preventing the accident.(cause i feel if the yellow haired asshole isn't stopped he would keep trying to get the FMCs sister).
REASON FOR SUSPECT
YELLOW HAIRED ASSHOLE - unbelievably nice behaviour, strange behaviour in front of the MCs brother(trying to show off that he is close to FMCs sister), falsely accusing the creepy ugly dude
GIRL WHO GOT HARRASED- her strange behaviour is very much expose her involvement in the stalking though I don't think she has any knowledge of the yellow haired asshole returning to past,also she was reassured when FMCs sister says that creepy ugly dude don't send the pictures anymore cause she felt guilty for framing him falsely
CREEPY UGLY DUDE- i think he's just what I call him
Creepy i don't think he would have harrased or stalked anyone.
WELL

THAT TURNED INTO A WHOLE PARAGRAPH
But i think that's the plot.]