The fact that you're married to her sister might be the only reason she's not interested in you and otherwise you're exactly her type - but she's not interested. I've found that when a woman's not interested, the time and effort (and expense, frankly) of changing that is never worth it. Maybe the younger sister thing makes it worth it for you despite the trouble you're buying yourself, I don't know.
There are a lot of ways this scenario could play out, and most of them lead to you getting a divorce and your wife's entire family, sister included, hating your guts and thinking you're a horrible human being.
Your best bet is to just think about her from time to time while fucking your wife and find something else to interest you. You wanted help, and honestly, this is the best advice I can offer.
If you're still determined to go through with this, your biggest problem is trust. Getting your wife's sister to trust you enough to sleep with you, while she's aware of the huge betrayal of trust that all of this entails, that's very complicated. Your second biggest problem is one of perception. The thing your wife is most likely to talk about with female friends and relatives is your flaws. They do this partly to vent their frustrations, the same way guys bitch to their friends about girlfriends and wives, but they also do it to give the impression that their boyfriend or husband isn't someone worth trying to steal. It's basically pissing on your territory to keep the other bitches away.
Your best bet is to defy expectations. Let your sister-in-law see that you're nothing like the way your wife describes you. Also, don't flirt with her, and don't try to make her feel good. A woman doesn't sleep with a married man because he makes her feel good, she does it because she feels bad, and she thinks she'll feel better afterwards. Sometimes it does make her feel a little better, and she continues to sleep with him to get more of that feeling.
She's your wife's younger sister. Unless your wife is the family screw-up, her little sister is going to have a lifetime of inferior feelings saved up, a desire to measure up to her big sister, years of resenting the things her sister got to do that she was too young for. That's your key, if you're willing to turn it. She's been competing with your wife all her life. Be subtle about it, but reinforce her ongoing failure to better than her sister, and resist the impulse to soothe her feelings when she reacts like you're a jerk acting as you are. If at first she pulls away and stops wanting to be around you, soften your touch a little. Tell her you didn't mean to hurt her feelings, you've just been under a lot of stress. Don't tell her you're sorry, or that what you said isn't true, just that saying it and hurting her feelings wasn't what you intended to do. You're showing her vulnerability by doing that without undoing your previous efforts. She'll still hurt, but some intimacy has developed. Be nice to her, but never overdo it.
Give it time, and eventually she'll start trying to demonstrate to you that she's a good person. Not out loud, not in such a straightforward way, but she'll start taking conversation in directions that make her look good to prove that you're wrong about her. At that point, you're looking for an opening, for her to say or do something flirty. Maybe she's got a new dress that's short and shows some cleavage and she asks you if you think it looks good on her... something like that.
Tell her you're happily married. Then tell her that she needs to stop flirting with you.
I know, I know. She's not flirting and she'll deny it and tell you you're crazy. Accept whatever she says, don't argue with her. Tell her as long as she understands how things are, you can both forget about it, like it never happened. Don't concede that she wasn't flirting, but emphasize that there's no need to make a big deal about it. She'll want to prove she wasn't, but she'll also want to wiggle out of this unexpected and uncomfortable situation as quickly as she can.
The point of that, of course, is to get her thinking of you as someone she could possibly have sex with. From that point on you're well on your way. Occasional subtle jabs to her ego, occasional praise of your wife, a lot of kindness to keep her feeling warm towards you, and enough attention from you that she knows she's someone you care about, and the rest will take care of itself. Eventually she will start flirting, which you should allow but not respond to at first. Go slowly and let her seduce you.
You're still better off letting this go, but if you're determined to do it, that's how it's done.