My gosh, I'm so sorry that you have NO support from your side of the family. I don't know what you should do but keep your head high and know that you are a better person. Don't let them get you down. So, your sister hasn't gotten a dress yet I don't know, if one of my sisters were treating me that way I would more than likely tell them nevermind and have my best friend as the MOH. It seems your sister should surely understand. After reading the IM it seems she may not be able to make it anyhow! I have two sisters and neither can afford to go but one made sure to save and the other can't come. My point is no one should be making you feel like your the bad person because you want to get married in Mexico. I know your FI's family is in Mexico. Maybe your family thinks you are only thinking of FI's family and not your side of the family. Regardless what they think they should love and support your decisions. Like you said, you're an adult!