Boyfriends mom blaming me?

I'm not sure how to phrase this. When my significant other was younger his mom had a very bad addiction to alcohol and drugs because she was sexually abused as a child. She got pregnant at 15 and eventually ended her relationship with his biological father. She has cheated on her current husband on multiple occasions which caused them to be verbally abusive towards one another. My partner holds a lot of resentment towards his mom and his family for causing so many problems and at one point even sought to take his own life and that's when I came into the picture. Him and I have been together for a little over a year and a half and his mother blames me for the way he treats her now. Apparently he shoved her into a tv stand and she cut her foot. "Every time he gets to talk to you he treats me his mother like garbage..." I can see where she'd get these ideas but Ive never encouraged him to treat her like he does. We've discussed his feelings and there's a number of reasons he behaves like he does. He doesn't trust his family, his mom particularly. I think he holds a lot of resentment towards her for not being there completely when he was young and she took up drinking again. I don't understand how to put all this. Can anyone voice their opinion on this? I don't think it's my fault but I'm not sure if it is my fault...