Would u b close to ur brother in law/sister in law who speaks poorly abt ur sibling (their spouse)?

My sister in law always speaks poorly abt my brother (her husband) to me & my mother. She will be disrespectful towards him in front of us. My mother and I tell her that she's lucky to have my brother and one day I reached my limit and told her that the way she treats my brother isn't good and it makes me wonder how she treats him when nobody else is around. I began to distance myself from her. I feel bad because when I was a teenager, I confided in her alot (although there were times she would put me down) but as I got older, the more it bothered me with the way she treated my brother. There's also the fact that I distanced myself from the Christian religion and she's a Pentecostal Christian who thinks every thing and everyone that's not Christian is evil. She would send me these emails about Jesus although she knows that I'm not a Christian.