My Manipulative Sister In Law

In high school I screwed my wife's big sister half a dozen times. When I first started dating my wife I didn't realize she was the little sister of my highschool fuck buddy. I was blind sided by this fact the day she took me to meet her parents. I ended up talking to her sister alone and we agreed not to tell her. Everything went well for almost two years, then one night after going out together her sister started to come onto me. Not lightly, she was saying let's sneak away and fuck. Saying she wants to suck my dick and sit on my face. I turned her down, and she got pissed, went to my wife and said she needs to take my drink ass home because I was offering to eat her out. My wife was pissed at me and wouldn't believe me that she was lying. Naturally, the entire situation pissed me off. I was furious with her sister for the lie, and pissed at my wife for not believing me. My wife goes on a tirade questioning if she could trust me. Because, in her words, why would her sister lie about something like that? So at this point my secret is one that could end our marriage. It blows over after a while and my wife, fully believing I said that, told herself that I did it pbecause I was drunk. A month later I was tasked with replacing the water heater at my sister inlaws. I tried like hell to get my wife to come. I said I'd take the next day off work and do it before she has to go to work. She said no, and that her sister really needs hot water and can't afford to have anyone else do it. I knew this was the start of a huge headache. I even toyed with the idea of setting my phone to record everything, so her sister couldn't get me in hot water again. Her sister does have a boyfriend, but apparently he is mechanically inept. I was stuck doing this it seemed. I tried to avoid her by going straight into the garage and getting to work, but about half way into getting the new tank in she came into the garage in short shorts and a pink tank top with no bra. Even though I hated her at this point, I still liked what I was seeing. I couldnt take my eyes off of her big tits. She apologized for getting me in trouble with my wife and said she was just hurt because I turned her down in the manner I did. She claimed I was very mean about it and it hurt her feelings. I already knew what was coming. She gets close and says let me make it up to you. I said and if I decline then what? She hesitated and said you don't want to do that, do you? In my head I thought, well fuck my wife won't believe a word I say anyways. If I get in trouble I might as well do something to get in trouble for. So I did. I fucked her right there in the garage. I sat down on the steps into the laundry room and she gave me a full penetration lap dance. When I said I'm about to cum she sat all the way down on my dick. She assured me that she was on the pill. Good side of the story she 100% left me alone after that. She became the nicest sister in law ever. Bad side, a couple months later she announced her and her now fiance where having a baby. It was a miracle!!! They have been trying for so long with no luck, and now they have been blessed with this baby!!! My mind spun wondering if it was mine. Well, the baby was born, and let's just say I hope that kid never does one of those family tree DNA test. Literally fucking no one in their family has blue eyes, but that little boy sure the hell does. Even has my cow lick and chin dimple. After comparing some of my baby pictures, I know someone else will connect the dots. I honestly don't know how no one has caught on yet. It's only a matter of time though. My wife and I haven't had kids yet, but at least I know exactly they'll look like when we do....

TLDR; Sister in law manipulated & blackmailed me into giving her the baby that her lame man couldn't.