Family reunion at a wedding?

I am going to my cousin's wedding in a few weeks and I am about to see my younger sister who I never talk to for years and years. We have a mom that does not know what is right and wrong and she likes my sister more because she is younger. Always has been that way and she gets away with anything. I am not on good terms with either one of them because they have made very shallow moral decisions with men. On top of that my sister has a very selfish, defiant character which comes from being in a dysfunctional family to a point where all my aunts think she has a pathological personality. There is a lot of betrayal issues in my immediate family which I don't think now is the time to address or heal. I love my aunts, uncles, and cousins who have reinforced all my family values and I don't want to seem like a hateful b-ch. What is the best way to make them know I am not on good terms with my mom and sister? I don't want to fake that I love them with all my heart!