There is nothing in the world wrong with you taking some time for yourself. There's nothing you can do for your sister or your step-father; they need professional treatment. How about just turning off the phone and giving yourself some time to just be. You can't be all things to everyone, but you can treat yourself to a break. Do something nice for yourself and let the other people (they are adults, so they are able to deal with their own issues) tend to their own stuff.
