I think that both during illness and also after the passing, my siblings and myself had very different reactions. I’m sure it’s just hard to understand each other and your emotions are all over the place. Sometimes people cope in good ways and others not so much. Like mhwolf said, you can try to help but its not so easy when they don’t want to hear it.
Family life is always tough during these times but you just have to hang in there and have faith that your sister will come through. If you really do think she’s self-harming though, I would definitely let your parents know!