1 of my friends is trying to take my other friend away from me what should i do ?

Well las year at the start of november i moved to a new school , i cam GREAT friends with a girl called stacey and she already had another close friend and i respect that so i didnt get to close because i though her other friend (sophie) would feel threatend that i might be taken stacey away but im not thats why im not too close , then sophie said last week why dont Me , Courtney , Megan and Stacey all go to M&D'S and we all said yeah and then sophie turned round and said , My mum could take stacey and megan but NOT u courtney so u will need to meet us there , I found this unfair so i didnt go , im not letting her treat me like that , and Yesterday she done the same with going swimming !!!! ALSO everytime i want to tell stacey something which is private SOPHIE comes over and says i have the right to hear this , and i say NO u dont its a secret and she still stand there , DONT say to me BACK OFF a little because i have and im sick of letting sophie say and do stuff like this ALSO by the way her birthdays this november and she has put who she is inviteing and guess what SHE HAS INVITED stacey and megan BUT NOT ME , please help with this !