I need some advice from someone outside the family

It's a month until we get married, and tensions have been building between myself and the parents of one of my flower girls. To start from the beginning, we had decided to have a super small wedding party: my sister, his sister, and my niece. Simple right? Well my little cousin calls and asks if she can be a "princess in the wedding." It was a tad obvious that the idea had been planted, but who can say no to a cute little 5 year old? I was happy to have her. However, everything related to the flower girls has turned into drama. "Why do they need matching dresses?" "Why do they need matching shoes?" Finally, they learned that we had scheduled appointments for the girls to get their hair done with us and things exploded. Her mother approached my mother in the middle of my shower to say she would do her girls hair. It was an inappropriate place for the discussion so my mom said fine. I was aware of the discussion and decided it was not a battle I wanted to fight, and was not going to say anything. However, her mother then calls my mom, hangs up on her, then gets into a shouting match in a second phone call. It's totally out of control. My mom tried to keep the shouting match quiet so I wasnt bothered, but someone spilled the beans. So now I'm not sure what to do. I feel that I need to take responsibility for my decisions, the fact that I want the girls to match and look nice. I don't feel that my mother should have to be the one getting yelled at. My mom, on the other hand, is fine taking the drama, and says its up to me if I want to call them and talk. My other worry is that they won't show up, and my uncle is the one that is doing the photography for the wedding. I should have eloped! lol