I think my stepsister is at risk for cutting herself, so I prominently display sharp objects in her room
That sounds dangerous. Why would you leave sharp objects where she could see them?
Well I think she might kill her self so I want to help her out.
If you're concerned that your sister may try to kill herself, then leaving sharp objects near her is a bad idea. If she has sharp objects at hand, she is more likely to harm herself, so make sure to keep the sharp objects away and do everything you can to support your sister through her mental crisis.
If my sister cut herself, everyone would be pissed and throw her out. I'm looking out for her best interest.
You need to make sure she gets help. You should tell your parents, she might hate for for it but it is worth being hated over having a dead sister.