I live through the exact same thing with my ex-inlaws.  They have even helped my ex stalk me as recent as two years ago.  I have been away from my ex for 6 yrs now.  My ex comes from a large family.  They all join in and bash me at family get togethers. 

Take a deep breath and truly take in what I am going to say.  What they are doing is Parental Alienation.  Research it if you haven't already.  The only way to combat it is...to do nothing but be the light to your children.  My exes family are very negative, horrible people who constantly have to put others down to make themselves feel better.  Most of them are narcissists and treat their wives horribly. 

Everyone says the kids will eventually see through it, and it is true.  You must take the high road and step back and view the world through your kids' eyes.  They see nothing but conflict and tension from both of their parents right now.  Take yourself out of it. How much better are they going to feel if mom's house is full of positive, happy feelings?  If mom doesn't react like dad and dad's family?  If mom doesn't have the cutting horrible remarks about people they love? They begin to feel your unconditional love, not just know, but FEEL it.  They begin to relax and associate your house with happy, warm, accepting feelings.  You have to change how you react.  It took me a very long time and I still stumble sometimes, but I have seen HUGE changes in my 14 year old who was completely closed off to me for a couple years.