Fiancé's in Jail, and I'm Convinced His Mom Hates Me

Ok. So 5 months ago, my fiancé was put in jail for doing something stupid. I know him better than most so I know had he been a sober mind, he wouldnt have broken the law. Anyhow, his mother HATES me. She makes it a point to call me and tell me her son is not my husband so I have no say in his trial, meanwhile I know I have no say, considering I'm neither a lawyer nor a judge. I've offered to go visit him with her and she has turned every invitation down and told me to go myself, only to later complain how she never sees her son. The final straw was at his last court date. I was upset with something that his family kept from me. I cried and got mad and asked that they dropped me home and not to discuss tho in the car. I wanted time to relax. She bashes on me, telling me I'm not family and in reality, I didn't have to know where they were sending "HER SON" for rehab. I simply turned to look out the window silently. Now get this. I'm not a violent person, and yet, she told my love that she thought I was going to hit her at that moment. As you can imagine, that created some friction. The fact she constantly states that he is her son shows me she is threatened by my presence. I don't know how to keep this strain off my relationship. I have always been respectful and kind to her, so i dont know why shes lying to her son about me and treating me like a plague... Does anyone have any advice?