Seating arrangment horror for wedding!?

I have a rather hard decision. I had my cousin as a bridesmaid and during an argument (unrelated to the wedding) she threatened to drop out of the wedding party. I told her she shouldn't be part of it because she has threatened me twice with leaving the wedding party due to unrelated wedding issues.I believe my cousin has a crush on my finace (which has contributed to her behavior towards me) and our arguments have been that I refuse to give her boyfriend a kiss on his cheek ( hispanic tradition as a greeting) because I have been informed that he has been verbally abusive to her. So, instead I extend my hand for a handshake. Well, we are not in speaking terms. My mother wants to sit next to my cousing which would be the first table closest to the sweetheart table.I dont' feel like doing that because I will have to see her, that won't be pleasant for me. My mother said she will sit anywhere as long as it's next to my cousin. What do I do??? The issue between me and my cousin are not about the wedding but about issues with her boyfriend and her little crush.