Replacing my Brother

My older brother was my father's favorite, he was named after our father, and was married. But they had yet to have a child; my brother was a drunk, a drug addict, and neglectful of his wife. I was his assistant and was actually running the part of the family business my brother was supposed to be running. My father wouldn't listen to anything against my brother. Our father's visits were few, brief, and known well in advance, so he had always pulled himself together long enough for each visit. I had one weakness: my sister-in-law. I had a crush on her ever since I first saw her. With my brother failing to get her pregnant, even her family kept asking when she would have children. Finally, my sister-in-law also got annoyed at my brother's lack of interest in starting a family. I just happened to be around to take advantage of that moment. So, started our affair, which has so far produced a son. My brother is usually out of it and doesn't know he isn't the father of his wife's son. I was just helping myself, but I also helped my brother. His wife is a willing sex partner for me, as she wants more children.