Convince my Mom to let me bring a friend to.....?

so im going to Marlyand (5 hour drive) this friday for my cousins grad party and we'll probs stay till thursday cuz my family lives down there. Im the youngest of all my cousins and the closest GIRL cousin is 18 :/....so often time i get left out when they go to see a movie, get their nails done, go shopping etc...and all my guy cousins and my brothers and play basketball, go to the arcade, see guy movies, etc....so often times i get left out of stuff. I REALLY wanna bring my bff with us on this trip! My mom would, but she thinks we r gonna be there too long. and its not cuz theres a party and stuff but my mom thinks our family is rly dysfunctional (ALL FAMILIES ARE!!!!)...idk i have no idea how to convince her! and the friend i want to bring is the most 'adaptable' of all my bffs..shes always ready to try new things and meet new ppl and shes very polite and proper....(the EXACT opposite of me haha)...but i rly wanna bring her and i need to convince my mom!! help!