SIL from HELL

Okay, so she isn't quite my SIL yet, but will be soon. But she is such a jerk to me ALL of the time. She talks down to me, she questions me like she's my mother, and literally tells me "you're stupid". She disrespects my fiancé in front of people, she tears him down and treats him the same way she treats me. She is an awful person. She does this to everyone but my soon to be MIL. She is such a nightmare. She treats my fiancé like dirt and I am SO over it. What do I do with this? I'm so ready to take her head off, but I don't want to cause tension. I'm over her disrespecting him and embarrassing him the way she does IN PUBLIC. Anybody ever been in a similar situation? Help a sister out!! Please!! 🙄🙄🙄