My Grandma Started it

A few years ago, my grandma then 61 (widow) came to live with us. She was getting very forgetful, and my mother 40 (divorced) decided it would better her mother live with us. Well off before and after the divorce and only having a bedroom cottage and my mother being a doctor put her mother in my room till another bedroom was finished being built. As my mother worked shifts it was better her mother shared my room. Two single beds and it was to be only a week to ten days at most. First night no problem and my mother working nights, I could've used her bed. But stayed in my room, I was awoken by my grandma early on Saturday morning about 1am. She kept saying my name, which was the same as my late grandpa. She was talking to my grandpa actually not me and she seemed to think I was him and she wanted sex and short version we had sex. She done it each night there after and then of course my mother caught me in my bed naked with her mother. At first shocked her 22-year-old son had fucked her mother. But not shouting at me so not to upset her mother, got me to go to kitchen with her. I told it was her mother who wanted sex and it made her happy, so I fucked her. My mother was calm and just nodded and then said some old women with dementia get aroused in the beginning of dementia and her mother must be one of them. As her mother was quite happy about living with us after first not wanting to live with us. My mother said she wouldn't try and stop her mother having sex with if I was okay with it. I was okay as I had been fucking my grandma for a week by then. But said it would probably stop soon anyway. Once the new room was ready and her mother moved in the sex had mostly stopped anyway. But she doe visit me every now and then, I wasn't great with girls but had my moments. But has I been happy to fuck my grandma, my decided I should fuck her as well. So, my mother took up the slack and that's how it is now seven years later. My grandma has passed the stage of wanting sex and my mother sleeps with me when wants sex and my wife doesn't mind and likes girl on girl action with my mother as well. We married early last year and we've a daughter born in May this year. Grandma still lives with us and is quite happy, but not really with it either, but loves holding her great granddaughter.