Why do family members baskstab others?

I just done get it. If you were to ask my husband and his brother who the sweetest, kindness, unjudgmental person in the whole world is. Who would they call first if ever in any seriouse trouble? Who is always there taking care of needs of each member of the family and never says one word? There reply wouldnt be me, or there mother, they would both say there sister.
WELL, I am not sure if she saves up all the backstabbing judgmental stuff for her friends or just for me but I have heard her say nasty stuff about each and every one of them. I have had her treat me like I am not part of the family and no one can do anything better than her. I have had her say awful things about my husband, there mother, brother, his kids, and my opinions. Not to mention her friends and husband. it must be hard for her. To sit back and never say what she is thinking avoiding an argument just by being pollite and pertending nothing bothers her.
I would rather be me. Tackey old me. That tells everyone what I think and never holds nothing in. Except with her I dont tell her how I feel. I think she is mean allot of time. She is mean to my mother in law and to my nieces. I think it is ridicules that she doesnt tell her husband when she disagrees with him.