Dear Lexie
I think it is clear that if they continue in this direction, it will eventually affect you directly. To be blunt, they will be short on funds, and you will be supplementing their income. In effect, you would end up having paid your step-siblings out of your pocket.
You seem to be one of the most level-headed people on this entire site. You are an inspiration to all of the rest of us, and one I would flee to for advice about matters like this.
Your situation sounds similar to my family's where my late parents were supplementing one of my sisters and her family. Of course, they had helped us all out from time to time, but my Mother kept paying my sister after my Dad died. Eventually my Mom got short on cash and I had to step in a little. Thank God I had it at the time!
Have you thought about setting up a trust of some kind? Would that be feasible economically?