My mother used to say “I’ve learned much since then & my story bears telling”.  I’ve had a toxic SIL and a couple of other toxic people in my life.  What I’ve learned is that the reaction your future SIL gave you is all about HER.  There is something deep seated in her that makes her lash out at everyone she comes in contact with.  I’ve found the best way to handle these people is to take a deep breath, not react, and try and look at her with compassion – that she is so miserable with her self and her life that she tries to make everyone around her miserable too.  If you can step back from it and not take is personally, understanding that SHE is the problem – not you, see her as a wounded human being deserving of compassion, it will help.  That and keeping your distance as much as possible.