It's always rough when people we care about are having trouble. I know that I immediately feel that it's my job to make everything OK, that I'm somehow responsible for other people's feelings. It's frustrating, though, when I realize that often there's nothing I can do but simply be there to listen and let them know that I care.
I don't know how close you are to your step-sister and your brother, but they're both in a lot of pain right now. There really isn't anything that you can do to help them except let them know that you are there for them. Don't beat yourself up for not being able to work miracles and make everything OK. No one can do that and this will only add to the stress your family is going through. Just let both of them know how you feel and that you'd like to help if there's any way you can and leave it up to them if they need anything.
This is advice that has helped me, so I hope it helps you.
Hang in there,
We are all in the gutter, but some of us are looking at the stars.
~ Oscar Wilde