How can I convince my parents to let me go to a leadership camp?

In April, there will be a leadership conference for a club at my high school called Key Club International. I live in Garden Grove, and this conference is held in Sacramento for our club division, which involves California, Hawaii, and Nevada. The conference is to help me as a Secretary for the club at my school and to teach me leadership skills, which will help me now and in the future. Along with that, I will learn how to work with others and gain social skills as well. The price was $375, and my parents promised that if I fundraised and made it less than $300, they would let me go. I have been fundraising, and now the price is only $250. However, my sister has convinced my parents that the price is still too high. Along with that, my mom said that she doesn't trust me because there will be boys on this trip also. My stepdad was one hundred percent for it and said I can definitely go. I just need to convince my mom and my sister to let me go. What should I do?