My friend acts different around me with other friends over. What should I do?

So, I'm 13, and I have this really close friend I knew since I was 4. Over the years, we met new people and we hang out with different groups in school. Usually after school (when school was still on), I would go to her house just to hang out then I would walk home. We would be a great match when we were alone with no one else, but when my friend is over or if her friend is over, we don't get along that well. So like today, my friend's friend (lets call my friend Camille and her friend Annabelle) came over today when I was over. Both Camille and Annabelle excluded me from whatever they were doing. Of course, we went in the pool, and when I got my "bathing suit" on (I really just wore rags I found out of my drawer because I don't feel comfortable about my body) and walked into the pool, Annabelle started whispering to my friend, and I knew it was about me. Since my friend has a sister a year younger than my younger sister, my younger sister came over (she's 9, turning 10 in November). I just chilled with my younger sister most of the time and her friend because the older girls were excluding me. So when I was walking by to get my towel (I was getting out of the pool), I heard Annabelle say quietly to Camille, "Why is she hanging out with girls 4 years younger than her? She so weird," that really got me pissed. And they knew why I wasn't hanging out with them! I really want to talk to Camille about Annabelle and how she's treating me, but then Camille would just get all defensive. I also tried talking to my mother about this problem, and since I go over her house when she's usually at work (my older brother and his friends always bother me, so I don't stay at my house) she said she'll try to talk to Camille's mother and tell her if Annabelle's coming over the day I'm coming over, Annabelle and her mother will have to make other arrangements because I have no other choice. Otherwise, I need more help. What should I do/say to my friend?