Responses
I believe no one knows? From what I understood of the spoilers others have given, the only one who kinda knew but is just as much in the dark was the blond hair brother. The black hair brother believes that she did it to destroy their family. It wasn't until the mother slapped him and told him from her mouth did he FINALLY believe that their father was the root cause of their family breaking apart. As for the youngest, his head is in cloud 9 and has no idea what's going.
I have to wonder do the brothers know the reason why their parents divorced, cause they mostly blame the FL for it, maybe the parents didn't wanna tell them to avoid conflict between ether parent, or maybe they knew the reason but still blame FL for helping their mom to divorce?