Mia, Ginger1 words are really wise. No matter what your H's family will stood by him, having said that it doesn't mean that they agree with what your H did. My SIL are tolerating my FIL's girlfriend and my other SIL' OM ( who is her H now), but bring their back wow, backstabbing is like a firework!
I know it is hard but you have to stand back and tell yourself that there is nothing you can do. Let it go and enjoy your kids.
