Tell her you want a good relationship with her... but also a good relationship with these girls.
Tell her she's really embarrassing you, and hurting your pimpin game (i wouldnt use that word though
) Ask her to please stop, then tell her that you would really value a conversation where you can talk with her about everything and sh*t you'll both come out with a win win situation.
If she wants to keep talking to your little girlfriends, kindly ask her to brag about how your "Peepee" hasn't stopped growing since you were 5, and how you were up to 7 inches by the time you hit pubert
And if all else fails, and she can't come to an agreement... in a tone that she will think you're joking but still think you might be serious tell her "well if you can't do that i guess next time i go to grandmas and grandpas i'll have to tell them about how i keep catching you with that same Fed Ex Man all the time!"
Good Luck