Grandma knows me too well

I'm James 25 and a electrician and doing quite well, no ladies man by any stretch of the imagination. Anyway my mother's mother Michelle 60 knows me very well indeed, having caught me a few times masturbating of the years. I had recently bought my own place, having had some luck and not being a ladies man I had saved my money. I like to bet, but not hooked on it and sometimes I hear of or get told of a good chance bet. That's how I had a big win that helped me buy my home. Michelle also likes a bet and was away when I got my great tip and so she didn't get to make a bet. So I don't get good info all the time and so don't bet all the time either. Anyway Michelle had a dream of winning horse and decided to go big on it. The horse didn't win and Michelle had used all her spending, housekeeping and power bill money to bet with. Now not able to pay the power bill or get the groceries, she was really caught. As William (Bill)64 my grandfather didn't like her gambling and only allowed her to continue as long as didn't go crazy as he says. Well she went crazy and knew once he found out he would let her bet again. That's where I come in, she came and asked me for a loan to get the groceries and pay the power bill and of course I loaned her the money. That's when the pandemic hit and Bill was told to stay home and so money became tight for them. I was still working as I worked at the local hospital as their electrician and was classed as essential. As time passed and Bill was unable to go to work and Michelle had promised to pay me back in a couple of months. That time came and went, I wasn't concerned as I knew she would pay me back as soon as could. But Michelle doesn't like not honoring her promises, so with lockdown finished and Bill still unable to work for a time. She came to see me and said sorry I've not paid you back yet. I said not to worry I'm okay with it, but she wanted to make up for it. Knowing I'm no ladies man and got off using my hand, she straight out offered me sex as compensation for the delay and adding Bill wasn't that interested anymore and it would also satisfy her as well. I was shocked and she was determined and we had sex twice that first time we fucked. I really enjoyed the sex with her and started getting aroused when she was around me. Once a week we got together and fucked and when Bill returned to work, she came over twice a week. I said you don't have to pay me back as the sex is a great payment by itself. Nonsense she said I'm no hooker and I'll pay you back and the sex is our special relationship secret. Knowing it wasn't a good idea to argue with her I just nodded. She has paid me back fully and still comes over for sex and when Bill went away for a weekend fishing with some friends she stayed the night and really gave me a draining.