How can I avoid hurting my sister's feelings?

My older sister is about a year into doing her beauty course and she keeps asking me to come into her class to have my nails and make up done for her course work. The problem is that she's really terrible at doing it! She did acrylic nails for me a few months back when I was going to a party and not only did she scuff my cuticles RAW with her nail file, but she put far too much acrylic on my nails and it was almost impossible to get off. I had to soak my raw fingers in acetone for hours just to weaken the acrylic enough to scrape it off. By the end of it, my fingers were so sore and throbbing I could barely use my hands :P I feel terrible because I keep turning her down, but I really can't put myself through that ordeal again. It's scared me off ever having my nails done, even in a professional salon. And I've seen her apply make up on people, and they always turn out looking awful. So my question is; how do I tell her I don't want her to do any of these things without hurting her feelings? :(