Friend issues I need help fast! sorry its long?

Okay So my used to be best friend "erica" Hangs out with her cousin "kelsey" all the time,"Kelsey" is a little stuck up snobby b**** and when "erica" is around her shes all bitchy. Its So annoying,Like she changes her."Kelsey" Is probably talking about me being my back to make erica hate me. now erica's grandma calls me and says Erica wants to play with you this weekend but she doesnt think shes allowed too because my mom flipped out on kelsey and not erica. So I said she can,as long as erica doesnt bring kelsey or anyone related to her up to my effing door. Can you guys give me answers about whats up with both of them??