You can't live for your mom and stepdad! Their health is their health and can't be impacted by you. You can only work on your health and it sounds like that would be best served out of there.

It might help to think in terms of maybe you will get stronger/feel better and in the future, be in position to help them again? Think like they advise on plane trips; get the oxygen mask on yourself first before helping children and others as, if you can't breathe, you can't be of help to anyone else.