Should I be my sister's best man in her wedding?

This is my situation (Its a bit long). My wife says that if I attend or stand-up in my sister's wedding, our marriage of 7 years is over. Her reasons, my sister show's favoritism to my 2-older children from a previous relationship (they live with their mother), and shows very little attention to our... show more This is my situation (Its a bit long). My wife says that if I attend or stand-up in my sister's wedding, our marriage of 7 years is over. Her reasons, my sister show's favoritism to my 2-older children from a previous relationship (they live with their mother), and shows very little attention to our daughter. Examples, my two older children are in the wedding and our daughter is not. My sister would leave compliments under the photos of my older children on my facebook and overlooks pictures of my daughter. My sister takes my oldest 2 children on vacation and does not offer to take our daughter. If she and my wife have a disagreement around holidays, my sister and mother will take back our daughter's gifts. My mother has called both my wife and daughter the wrong names for over a year. My sister and mom never pick up the phone to call my wife to speak to our daughter. My wife thinks that they don't buy appropriate gifts for her and our daughter while she buys all of my family gifts every holiday season. Example, my mother and sister would buy my brother's girlfriend designer hand bags and get her slippers. When my wife confronted her about the favoritism, my sister went off and said things that were inaccurate.

The other side of the situation is my sister says that she did not include our daughter because she is 4 and probably will have trouble following direction. My wife has relatives that have been mooching off of us ever since I could remember. This includes constant begging for money, a place to stay, cigarettes, a ride somewhere and so on. She has relatives that have problems with addiction and have made some scary scenes in front of our children that I don't think should be tolerated but she still keeps them around. My belief is, because I have put up with years of direct and indirect inappropriate behavior from her relatives without ever giving her an ultimatum (because I belief that most in-laws are screwed up) to have nothing to do with her family, she should have the same consideration. HELP!?

By the way, did I mention that we're an interracial couple? She's Irish/Italian and I'm African American.
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