Grandma moves in

I'm Alan 20 middle child of Wendy and Wayne, my older brother Gordon is in the navy and overseas. My sister Gwen 15 is into sports and regularly plays sport on the weekends. My mother's mother Ruth 60 moved in with us April after grandpa Gordon passed away in late March. Both my parents are busy with their business and I was alone most weekend till Ruth moved. I'm a carpenter trained by grandpa Gordon when I left school at 15, I use the large shed the back of the family five bedrooms 2 bath ranch-style house. I use it to make custom doors and window frames for clients. Grandpa Gordon tools were all given to me by the extended family after his death and I installed them into the shed. Back in May, I was working in the shed on a Spanish style door for a client and Ruth came out with coffee and cake for us as a snack. While eating we talked and she asked me would I drive over to see her old house and then to the cemetery to put flowers on Gordon grave. I said okay I'll just glue and clamp the door and we could go. We parked outside her old house and she told that Gordon had built by hand on weekends before they married. Then we went to the cemetery and visited Gordon's grave, after as we were on the other side town from home. She asked would I take her to the old lookout for a while, so we went to the lookout and while sitting looking out over the town and countryside. Ruth told me that Gordon and she would come there and have sex. I looked at her and she asked would I fuck her one last time there. As the lookout was being removed and a phone-tower installed there soon. I was shocked at the request, mainly because I didn't think she had sex anymore and I'm her grandson. Seeing shocked expression she added that I looked so much like Gordon my age and she missed their sexual intercourse. I'm not the greatest looker and I think I do okay with girls I date, but nobody special yet. Ruth isn't bad looking for her age and my cock thought she was fuckable and got hard straight away. I said okay if you really want to and we had sex, this was the start of a regular sexual relationship. When we were alone at home she would ask me to fuck her and then I started asking her, hardly a week goes by I don't get sex from Ruth. When my parents went away last month for a holiday and Gwen went to uncle John and aunt Christine for a week as well. They own a farm and Gwen went to visit our cousins and ride horses. I slept with Ruth every night. We bathed together every night as well.