I'm really interested in how she knew since she seemed to be the one who knew the least not the mention the whole thing about her first time. Unless that is all a lie she learned from her sisters and was trying to protect Sasae even knowing he was the one who killed the father. Or possibly she only learned the truth about him after he died and she was looking into the murder and realized he only had enemies and was a piece of trash abusing his son, stealing from others, and being a general threat to women