It started because of a party

I 27 had been working late and got home and there had been a party and it was breaking up on my arrival home. I wasn't concerned about there being a party as it was the usual Saturday night thing for my sister 30 to go too or hold a party. We lived together as it was cheaper to share a place than each have our own place. My sister was clearing up the dishes and getting rid of the rubbish. She was drunk and couldn't walk straight at all, but she persisted getting most of the rubbish cleared away and dishes in the kitchen. The last of the guest had gone by then and she sat down and had another drink. She was between boyfriends at the time and had overindulged as a result. She started playing with herself and she didn't seem to notice my presents. I got horny watching her and decided to give her some help. She didn't try and stop me, and I was soon sucking her tits and fingering her. This really got me really aroused and I asked her to let me fuck her, she mumbled something I couldn't understand and took as her agreeing. Not wanting to fuck on the lounge room floor I took her to her bed as it was closer than mine. I fucked hard leaving my load inside her and after a rest fucked her again. I knew her to be on some sort of precaution and thought it safe to cum inside her. I knew was a bit of a stretch, as I knew she used precautions dating her boyfriends. But she wasn't seeing anyone at the time, and I assumed she was still taking precautions. As it turned out her precautions had lapsed, and she had updated herself at that time. She awoke in the morning with my sperm leaking from her pussy and me asleep beside her. She already knew I had fucked as she encouraged me while I was fucking her to go harder. It was after breakfast that it dawned on her she might get pregnant, but still wanted me to fuck her again, but I had to use a condom. Sex between us was regular after that and she of course got pregnant, no worries she got tested and with every test showing no problems decided to have the baby. The only change was no condoms while she was pregnant. The baby is due in late May or early June. It's a boy.