Move in with my widowed sister

I'm the youngest of 4 and on finishing university I got a job in the city, I hadn't found a place to live at the time and planned to book into a motel while finding a place of my own. My only sister 32 the eldest of the 4 was widowed late last year and had a daughter 4, her husband rolled his truck, and it went down a gully and he was underneath it when it stopped rolling. They had their own home with a bank loan still, my sister was trying to pay off the loan as quickly as she could with insurance payout and work as much as possible. But she was just holding her head above water as they say. My mother told me to move in with my sister and pay rent to help her out. My sister and I weren't on the best of terms growing up, her referring to me as the pain in her life. But absence makes the heart grow fonder as the saying goes and now being around each other very much over about 12 years made agree. Anyway, I moved in, and no hassles and I done my thing she done hers. I liked playing with my niece and started watching her while my sister worked, whenever I could. Then early last month I was in bed reading and almost ready to go to sleep, my sister came in and she was drunk, she had been drinking when I got home, and I didn't say anything. She rarely drinks and every now then she as a drink and then sleeps it off. But that night it was different she was horny, and I was the only male available to her. I'm not a great womanizer but have had my moments. Anyway, we ended up fucking as I was soon horny myself with my sister dropping her robe to the floor and being naked. I got aroused, in the morning she was sober and feeling ashamed of her actions and I cuddled her, and we ended up fucking again. Ever since we been fucking regularly, my sister has missed her period and she is a little worried at the moment. But I'm willing to wait and see and there are tests for everything now. But the hoodoo on sex between siblings for us doesn't exist anymore. I'm quite to fuck anytime she is willing, and she is willing. Whether she is pregnant or not I'm willing to continue our sexual relationship.