Oh my. I'm so sorry. I guess the question I have for you is if your sisters were put into safe environments, even if separated, would they be better off than they are right now? I personally think they would. They are all being abused, just like you were.
I think you should consider fully disclosing to your therapist and asking for help on how to help your sisters. Additionally, I think I would find other arrangements for your son. Does the college that you attend have a daycare center? If you are lower income you should be able to get a lower fee.
Finally, I just want to say that you are doing an awesome job of getting out and moving on with your life, being a wife, mom, worker and student. That's is truly great! Keep moving forward, and don't let your Mom, step-dad or grandmother stop you or interfere with that. You can be a role model for your younger sisters and your son.