I'm REALLY mad at my mil, please help.?

So, My hubby's little sister just broke up with her b.f. of five years because she "fell in love with another guy". She was cheating on her b.f. w/ this guy for like two months. Okay, she is young and stupid, I'm more mad at her mom for not reprimanding her. Her mom, my mil, acts like she is totally justified in doing what she did and like it's no big deal. I'm so furious, I feel like she has failed in so many other ways that this is just the icing on the cake. If anything she was like, celebrating the fact that she was with this new guy with her daughter. I can't believe that this woman has no moral or ethical backbone with which to impart wisdom on her daughter. I'm ashamed that these people are my family. Am I overreacting or should I feel the way I do about the whole situation. Should I say something? Her daughter is still young and impressionable and I wish someone would tell her it's NOT OKAY to just go around and cheat on people.