You're not a terrible friend. I'm sure you've done all you can to help them change their destructive thinking. But, it's really hard to change people that don't want to listen or be changed. The parents in both cases need to be involved...it is pretty sick if they are treating it like a joke. Maybe a school councellor could help them (but getting them to see a councellor sounds like it wouldn't work). The most you can do is hang out with them and do activities that are fun, this will get their minds off their problems at least for a little while, and maybe it can become a habit for them to seek out fun stuff.
TheMarcherFeatured By OwnerDec 8, 2012Hobbyist Photographer
It is very hard to share feeling about self harm to people who are so close. It might be a better idea to have your sister talk to someone else. Perhaps not a doctor but just a friend.
I know I could never have told my mom or my sister about what I felt back in the days when I self harmed. But I could talk about it with strangers. I did not feel the need to keep a facade when I did not know the person well.
And you are strong, you can be strong enough to face anything. Even if it does not feel like it you are always able to take another step. Just one at the time, and then you take one more and repeat until you are done. And you don't have to do it alone. There is plenty of good people who can help!