Primary caregiver is my step dad. We are driving each other crazy! Any advice?

I moved in with my mom and stepdad 16 months ago ( I was 51). They have been together 27 years. I have always been close with my mom but I was grown and on my own when they married so not as close with step dad. He is extremely controlling, impatient and has anger issues. His sister comes once a week or when he has a doctor appt. She has also come to spend the night every 2-3 months to allow me some respite.(Which is the only respite I get. My step dad refuses any respite.) His other sister brings food twice a week. My sister had a temper tantrum and refused when I asked her to help. I am reading books and seeking counseling from the hospice social worker. I went back to school before I moved away from the city to be closer to them. I searched for a year when I was here in the "country" and then after I moved in with them I searched for another year for a position in my new field. I continued to wait tables until I finally found a job in my new field 3 months ago. Since then things have gone downhill between my step dad and me. I talked to him repeatedly about what my schedule would be, how many hours I would ask for in my cover letter. He just said "get the job and worry about that later". Now I have the job and I tried to discuss my schedule with him several times but he just clammed up. The tension and criticism is really distressing. I would never be in my parent's home at this age except for the fact that he asked me to move in because he needed the help. I feel trapped and because of my love for my mother I can't escape. I have thought of moving out, coming to their house after work getting dinner, taking care of toileting, and getting mom to bed then going to my own place. I would also come each weekend from Saturday morning to get mom out of bed, stay till bedtime, then do the same on Sunday. I would do that 3 weekends out of the month and take one weekend a month to myself. It sounds reasonable to me but I fear that he would shut me out if I move. Of course, he is already shutting me out emotionally with his passive/aggressive hostile behavior. Don't even suggest talking with him, that is impossible for him. He sincerely believes that there is nothing wrong with him and that everyone else is "an idiot or a ding-a-ling" I can't take this much longer and I sure can't take it for a number of years. My mom is last stage ALZ but there is no predicting how long she will need me. Please give any advice, suggestions, or ideas especially if you have been there and found a way to survive!