It is awful, I think it is worse for parents and siblings, at least that was my experience. My son tried it twice, luckily he didn't try very hard (his natural aversion to pain stopped him). I found he was quite matter-of-fact about it.
In our case I didn't see it as a suicide attempt, he wasn't even particularly depressed when he did it. It was a response to a very stressful situation, and I think it was a way of releasing it for him.
Perhaps you have been told about alternatives (rubber band flicking, ice). I think you could probably have an honest talk to your girl and say "It is distressing for your sisters, can you try an alternative?" It sounds like she is concerned about scaring her sisters, she might be willing to try something. Obviously telling her to just stop isn't going to work. As for protecting the house, I put away all the potential methods of self harm I could find. I locked the front and back doors, and insisted that the bedroom door stayed open when he was asleep. I tried not to go overboard - I didn't want to add to his stress.
I found with my son he was open to discussions about keeping him safe.
With my son, I offered activities: swimming, basketball with me (he likes to beat me), walking in the park. Often I would match it with a "reward" - go for a swim and I'll turn on the wifi...etc. At the same time, no stress. If he really wanted to just lie in bed, I let him. I did try to maintain a sleep schedule, so I'd limit his time asleep during the day (20 mins I remember, then I'd wake him up).
All kids are different and hopefully you can get some advice from a psychologist soon. I did find I had to work out the details for myself a lot - the psych can't tell you everything.
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