How do I care for my grandparents when it was supposed to be my mom's job but she dies 2 years ago?

My mother passed away 2.5 years ago at 52. Her death basically left me as the soul caregiver for my elderly grandparents (taking them to dr. visits, visiting a couple times a week, helping around the house). My grandmother is 84 and in reasonably good health. She still drives and can get around. My grandfather (87) however has had a series of strokes over the years, with the last one being about 5 years ago. The last stroke left him with very limited movement on his right side. Up until recently, he had still been able to get up and walk (either on his own, with a cane, or with a walker). For the past week, other than one time, he has been unable to get up from his chair (other than just standing for short periods of time). My grandmother cares for him most of the time. He's incontinent and is in adult diapers which my grandmother struggles to change (she only weighs about 90 lbs). I've been over to their house for several hours a day since Friday to help her get him up from his chair. There is no way she can lift him on her own, despite him having a powerlift chair. I'm at a loss. He doesn't want to go into a nursing home, but he needs more care than my grandmother and I can provide. Other than SS and his pension from the army, they don't have any income. They have medicare and he has an HMO, but I have no idea what services are covered. I'm starting to feel very resentful (and I hate it) of the amount of time he expects of me. He will ask me almost daily to stop by on my way home from work. Over the weekend, I had to leave a movie early because "he needed me to come help him get up" and then after spending 3 hours there that night, he called me first thing the next morning to come back. I want to set a good example for my boys, that you care for the people who have cared for you, when they are in need, but I don't know how long I can keep this up. I just ordered him some condom catheters and am hoping they will help make him more comfortable (the diapers leak and he ends up wetting himself) but I am struggling with what to do about bowel movements. Even if I can get him into his wheelchair and to the bathroom, he can't walk the 5 ft to the toilet. I'm looking at portable wheeled commodes now that could be put in the living room near him. I would still have to be there to get him on and off, but it's a start. I welcome any and all advice. I'm so glad to have found someplace to ask questions and vent!