My family thinks me and my guy cousin pick on the girl cousin?

I am 15 and everytime I get together with my cousins something bad happens Usually it's stupid things. One time in the mall. Me and my cousin made up a code word but everytime we tell the girl she give it all away by making it obvious so obviously we didn't want to tell her So my mom jumps in and starts yelling TELL HER TELL HER. So we told her and she didn't belive us. End of that story. Also the girl cousin is becoming a brat because she knows she can talk back to anyone and no one can get mad at her. She is on broadway so I think that makes her think she doesn't have to respect any adults Another thing that happened was we, the three of us were in my room talking about people that we liked. Out of the clear blue sky my mom runs in and said leave her alone to me and the guy cousin! We had no clue what she was talking about. We tried to ask but she was too mad and kept saying " you know what you did !" EVEN THE GIRL cousin WHO MY MOM WAS "DEFENDING" didn't know what we did wrong! Another thing that happened was we played find the Easter eggs. I didn't really want to but I did and my aunt hid the eggs and was telling where they were to the girl cousin. I know it's not really I big deal or anything but it just shows how my mom, the aunt and the girl cousin are a team. And my mom always trys to say how me and the guy cousin always gang up on the girl cousin. We got fed up and told the grandma about how the mom and aunt and girl cousin act. Btw my grandma is the best. She's the only one who isn't sexist and she actually listens to our stories instead of attacking us about nothing. Me and the guy cousin can't even have normal conversations about it because my mom always jumps in and blames it on us. I need some serious advice because now I am pissed of but I can't do any thing about it. I could thing of literally hundreds of stories from my life like this but I won't. Just give me advice. Don't say something dumb cause it makes you look stupid. Just help