didn't she herself say she only needs salvia (or blood) from him to survive? Couldn't eat normal food either, saying it's gross. It could be a psychosomatic reaction and her anorexia, but they do mention that the pills kill the hunger (for human meat). Also, no way would big sis go that insane for the taste of human meat
even if she were crazy, she acted as if she was starving.
Also, I worded it that way above because I didn't want to spoil the ending, but it's true, in no way do they need to eat actual human meat, but they do need to eat something from humans, either way.