Incendies 2010!!!! what the fuck

[SPOILER!!!] WTF!!! bro during the scene when the guy says does one plus one can make one and the girl screams i didint knew what did just happened i didint knew what did it mean but i got some serious trauma while watching that scene i just knew something really terrible was revealed and at the swimming pool scene when she was slowly reaching him i was thinking dont tell me the one who raped her was his own son ! and when i saw that guy i failed to recognize him but then it was revealed that i predicted it right and the last scene when she says that you dont recognize them but they know who you are that fucking killed me and those two letters giving two perspectives of dad and son i mean whaatt was the director thinking man i mean what!!!!

ps- can some one explain what does can one plus one can make one mean ?