You and your cousin are basically besties. You don't need to seal it with a pinky promise, or make a pact for life. It's just kind of this known thing between the two of you. She's your main squeeze, and you really wouldn't want it any other way. The embarrassing things you do in front of your cousin, in particular, are probably what make you so close. Let's be honest: You wouldn't have those moments with anyone else. *Blushes even at the idea of it.*
Yes, I suppose if you want to get technical, she's your family member, but she's also your favorite one to hang out with. (Don't tell the others, OK?) Growing up and all of those holiday get-togethers would've been so different without her. She taught you that the slices of pizza are truly the best appetizer, and that since you're the youngest, you can get away with asking to open presents before dinner. Truth is, she's always considered you her sidekick, too.
Even growing up in different states didn't really stop you from becoming the best of friends. You and your mom would road trip to her place for barbecues in the summertime, and now you and your cousin are planning trips of your own. There are so many more memories to be made, am I right? Just be prepared for these seven embarrassing things you'll likely do along the way. It's OK, because she's your bestie, after all.