Am I handling this family situation the wrong way (please read!)?

My sister in law (brother's wife) is very spoiled and mean to me and my sister. She is unfriendly, cold and bad mouths us to others to the point her family and friends don't like us and she is turning my brother against us. My sister wants nothing to do with her,but I am nice by nature so whenever I see her I act nice, which my sister interprets as needy and kissing up to her which allows her to continue walking all over me like a door mat. Yesterday I was nice to her when I saw her and posted comments on her (sister in law's) facebook, so now my sister is fed up with me and is not talking to me, saying I am too much of a people pleaser even when others are clearly abusing me and hurt my feelings and lie about me. I told her I'm just trying to show others I'm really nice and that the things she says about me aren't true, but anyway my sister has lost respect for me and now I can't get her to talk to me (she'll eventually come around, but am I handling this wrong? Should I ignore her and give the in law attitude right back?)