How do I handle my mom in laws instigating ways?

The problem is that every single time me and my fiance (her son) has a problem, or if me and his sister has an issue, she always like to get in the middle of things and manipulate the situation. Also, of all the problems that her grown kids have or had, she's always harder on me than she is on her children, also to mention that I'm 22 yrs old. She also repeat to me negative stuff that he or she said about me, but will deny everything when confronted. The thing is that I don't want my relationship in jeopardy based on my feelings about my bf's mom. What are u guys opinion about her and should I stay away from her?