Facebook probably gave your cousins each other's names (as "suggestions" or as your-friend-has-these-friends-why-don't-you-befriend-them-too and, maybe, one has the same last name as yours? Maybe they play the same game and just need another "body" to add to their number; I don't think it is always going to be about us.

Obviously your ex-friend doesn't know how much you "hate" her, why don't you tell her again?