If you haven't seen The Dreamers and you plan to, look away now.
Earlier on she did say if her parents found out, she would kill herself. They found out, so she tried to kill herself and her boys. They have been so caught up in their own world, they have been totally unaware of the situation exploding around them. The rock coming through the window is a wake up call and it's reality crashing in on them. They go out to see what's going on and when Matthew realises that he will never truly have Isabelle, she will never split from Theo, he walks away. The end.
Nothing precious, plain to see, don't make a fuss over me. Not loud, not soft, but somewhere inbetween. Say sorry, just let it be the word you mean.