One Mistake is all it takes

Last year in May I went out drinking to celebrate my promotion with friend getting home just after midnight, not working for 2 days as it was Friday night. I'm David 31 and I live my sister Miranda 28 nurse. She got a job at the local hospital and move in with me on arriving in the city. I bought my 3 bedroom house on starting work after finishing university, still paying it off. But it gives me freedom and I don't have really close neighbors. Miranda isn't into the dating scene and had been working long hours due to the pandemic and shortage of staff. I was still in a party mood when Miranda came home from her latest long shift. She entered the house just after me and I wanted to party on with her. She had a quick dance with me but mainly wanted to sit down and and a coffee. Anyway we sat together and I got fresh with her and we ended up in my bed fucking, I think she gave in hoping once I was satisfied she could some sleep. She did but the damage was done, working all the time and not dating. Meant she hadn't taken precautions and was too tired to be concerned also. I didn't precautions myself as I didn't think of taking any at the time. In the morning we had sex again this time I used a condom, but it was too little too late. Miranda had thought about our first fuck and I just wanted more sex from her. The first it dawned on her we might have made a mistake was missing her period and decided to get tested after missing her second period. A positive result brought back to reality, I had gotten my sister pregnant. She told me I should tell our mother Sharon 52 war widow her only son had gotten her only daughter pregnant, to say she wasn't pleased is an understatement. But after she calmed down she sold her home and small business and moved to live with us. I had been using the pregnancy of Miranda to keep fucking her. I usually get my way with Miranda and always have, on mother's arrival Miranda started wearing her wedding rings and Miranda changed her status on her work profile from unattached to married. Not that many people at the hospital had time to notice over the time she was working there due to being busy with pandemic. Sharon told me I was to act like Miranda was my wife and not to party as before. Luckily I moved offices because of my promotion and hadn't interacted much with the staff much at the new location. So when Miranda and Sharon dropped by to get my car as Miranda's car was in getting serviced. Sharon told the receptionist she was my mother and Miranda my wife and they saw me got my keys and left. So now I was supposed to be married and I've enjoyed the experience, for one thing Miranda shared my bed and becoming a father to my beautiful baby daughter. With Sharon living with us taking care of her allowing Miranda to continue working.