I got my Mom Pregnant by mistake

It was last christmas holidays I was home from college and I was staying in our guest room over the garage. As we had alot of relatives visiting. Guest room that stretching it. It was attic on top the garage used for storage. It had a old bed and a tap with a basin with a small window about 14 inches square. You got into it by an outside staircase and I had to remove cobwebs and dust and spray 2 cans of air freshner, before it was barely good enough to sleep in. Mom made up the bed with new sheets and a new pillow. I had always got on well with my cousin Ruth and had fucked her a coupla times.
I went out on christmas afternoon to visit some mates, but had made arrangement with Ruth for her to stay with me that night. While I was gone Ruth got a severe pain and taken to hospital, it her appendix. Also Mom & Dad had fight after drinking all day and Mom went to the attic to sleep. I came home around midnight and had a few drinks myself. When I got into the attic I saw a lump in the bed and thought it was Ruth. I got undressed and got into bed and started rub her slowly. I put my hand down her panties and rub her vagina, fingering her. She moan a bit and pulling her bra off her tits I started suck them. I remember thinking her tits were bigger than I remembered. I then removed panties and went on to fuck her. She only moaned softly and never woke up. I woke up first about 6am I needed to go for a Pee. I dashed out and when I came back, there was more sunlight lighting up the attic and I could see I had fucked my Mother and she was still a sleep with a smile on her face. I grab my clothes and ran to my car and I went to a diner and had a breakfast. About a hour or so later I returned home to face the music. To my surprise Mom was happy and cheerful and she slap Dad on the bum and winked at him. I could see Dad was a bit puzzied, but he didn't say anything and later they were in their bedroom for an hour and Mom came out still happy. About 10 weeks later Mom told me that she pregnant. She said Dad must have busted a condom and hadn't noticed. But they weren't worried as they thinking of having a fourth child anyway. That how I got first Sister Joyce.