HAHA I KNEW SAYAMA IS NOT THAT KIND OF PERSON.
At the beginnig of the story I already have a hunch he had that kinda traumatizing backstory.
But for real tho. If i was him, I'd not forgive his "sister"!!!
I mean, think about it.
Sayama is only a 12 yo at that time. He's still practically a child. Yet she reacted like that. Jeez.