Whats Your Opinion about Backstabbers?

I have been wondering lately if my sister in law is a backstabber. She has been my closest friend since we moved to Pennsylvania 4 years ago. But recently I noticed that some of the things she says don't add up. Also if I will tell her about something that is going on between me and my husband she will completely side with me, but if my husband talks to her about it he will come home and tell me she thinks he is right. I don't know if I should confront her or let it go because we are moving out of state soon so I will not have to deal much longer.

People generally have to earn my trust. If someone stabbed me in the back I would never trust them again.

I hate when a person stabs you in the back. I am not a person to just let it rest either.. I will call them out on the things they have done and then be done with that person.