Babysitting question? Please Help!?

I agree that you are in no proper frame of mind to be caring for any 20 month old, be grateful it isn't yours, and you should dump that responsibility right back on your sister where it belongs. Mental health problems make it very difficult to make decisions and act on them in the way that is necessary to assure the safety of a child that age. What energy you have you need in order to find and do a job that pays a decent wage. That is your right. Your mother went way over the line committing you to be the unpaid slave to your sister's kid. You will simply have to tell them both that you may occassionally be available, with advance notice, to sit for the baby but you will be interviewing and pursuing a better job and you must have control of your life and be able to make and keep appointments for that purpose. Do not take no for an answer. They may be pissed at you but if you let them continue to use and abuse you will have no life at all except in a mental health ward.