It always seemed to me his sis did 'drop and run’s on him. The daughter of my father’s wife used to do that to me. “Going to the store-- be right back!” and be gone half the day. I understood Donald’s grumpiness. Or am I projecting?
You aren’t projecting. Some people will do “drop and runs” if they possibly can. Until you start charging for babysitting. “Let’s see, that’s three kids, at X per kid, times Y hours, so you owe me $45.” 
Works like a charm every time. Now, you won’t get PAID, but it’ll be the last time you get imposed on.