Sister's Mother-in-law

My sister and her family moved interstate as her husband got promoted and they had to move. But her mother-in-law (widow)53 couldn't go as well as she had a thriving little business. But as she used to get her son (only child) to move things around for her, meant she would've to either hire someone or move them herself. My sister then thought of me her younger brother and only sibling. I was living and working locally and had been getting Sunday dinners at my sister's home for a while. Suggesting I help her mother-in-law and might just get Sunday dinners with her. Anyway, I agreed to help if needed, I 27 work shifts and didn't have a steady girlfriend, I dated and got lucky, but nothing seemed to go any further after a few dates we would break up and I would find another temp girlfriend. Then everything would repeat as the previous relationship, I already knew the mother-in-law and so no problems there. A week after my sister and family had left, I was duly called to help, which I did. Turns out sister's mother-in-law (Margret) liked cooking and making a Sunday dinner wasn't any trouble for her. But she didn't bother if she was to be alone, having dinner with me she happily cooked every Sunday. No problems at all, back in September was cleaning out the garage, I was helping her. Told I could have anything I wanted of her late husband's tools what were left after her son had had his pick. I got somethings and while shifting a stack of boxes, a can of thinners on a shelf was knocked off and hit a bench. Lid came off and I was splashed with the thinners, I was in coveralls and only had shorts underneath. I opened the top of the coveralls where thinners had wet the coveralls, to get it away from my skin. Margret said to remove them completely and she would wash them for me and told me to shower just in case. So, now in my shorts I went to the bathroom to shower, after getting in the shower Margret entered with some clothes of her late husband as we were about the same size. The shower screen is clear glass and hid nothing of my body, then without saying anything at all first she got naked and joined me in the shower. Saying as she joined me, she would make sure my back was washed properly. I was at first shocked but soon got aroused by her being naked in the shower with me. It was easy for her to see my arousal, so she started playing with mu erection. I fucked her in the shower, and we spent the rest of the morning in her bed. We have now a mutual satisfaction type of relationship, sex when one or the other wants it. I really mean when one or the other wants it, I've asked and she has never refused me and she has asked me, and I haven't refused her either. I'm always willing and get horny every time I see her.