Need to know my rights as an employee. Being taken advantage of, not able to leave to care for elderly mother, etc.

I have worked for this company in the travel industry for 10 years. Admittedly, I have allowed myself to be taken advantage of during this time. I am salaried, and typically work 10 to 12 hour days - sometimes longer.I had a heart attack a few years ago, and for a time my employer eased up, but that stopped and things returned to normal. Normal being excessively long days, additional work assigned so they can leave early (while I stay even later), and the bosses being verbally unprofessional. Recently my circumstances have changed where I am now the caregiver for my 94 year old mother with late stage Alzheimer's. I need to be home before 8pm so I can spend some time with her, feed her, and clean her up. But my bosses still won't let me leave at a reasonable time. What are my rights here? If I email my request/need for a typical 40 hour week and just start leaving after 8 hours (with my work being done) can they fire me? Will that documentation help me get unemployment?