What do you think I should do?

I am in the cast of the show Oklahoma at my high school. I am cast as Ellen, the farmer's daughter with 4 lines and a solo line in Out of my Dreams. (What a big role!) Our show goes on Dec 4, 5, 6, and 7th. The director did not make understudies but I was kind of designated Aunt Eller's understudy once Aunt Eller started missing a lot of practices due to sickness and marching band. It's now 3 weeks before show and she has two nodules on her throat and she was put on vocal rest (for a period of 2 weeks) one week ago. She now has the stomach flu too. She hasn't been in school now for 4 days consecutively and I'm starting o think I should memorize her lines. I don't want her to think that I want her to stay sick or not be in the show after all she's a senior and I am a sopre. I am happy she got a more main role than I. I don't want her to think I'm betraying her or anything...what should I do?