Duet for dance w/my sister...?

Me and my little sister Holly REALLY want to do a duet for dance this yr. We are really close and I just think it would be SO much fun. The only thing is its really expensive...and my family doesnt have a ton of money. so how can we convince our parents that we really want to do this?? I offered to help pay but i dont have a lot of money myself either. and YES i am getting a job, but i'll have other things i'll have to pay for too like my car, etc. and this is going to be my senior yr so i'll be busy with that too....but i really really want to do this with my sister. especially sense ill be going off to college nxt yr and this will be a way for us to bond and have fun while doing it before i leave. please give me any advice you have! thank you!!!