No, you just can't do it.

Despite the blood that had rushed to your cock, there was apparently enough left in your brain for you to think clearly and you scooted yourself away from her and stood up from the couch. "Kelly, I can't believe I'm saying this, but we can't do this." you tell her in a gentle tone. "I know you're hurt and angry. I'm angry too, but us going through with this won't help with that, and, if anything, may ruin the relationship we have together. I'm closer to you than I've ever been with him and I don't want that to change just so we can have a **** fuck... even though it would probably be amazing." you said, chuckling, in an attempt to lighten the mood.

"Ugh fine, you're probably right David." she said, grabbing her sweatshirt and slipping it back on over her head as she climbed to her feet. "I guess I probably shouldn't fuck any of his friends either. Honestly, none of them really do it for me anyway. You were the only one who really peaked my interest." she said, reaching up and giving you a quick slap on the ass with a wink and a mischievous grin.

You couldn't help but laugh at her playful attitude and found that within only a few minutes, you felt as comfortable around her as you ever had without any of the awkwardness you would have expected. You left a short time later, promising that you'd stay in touch with her and letting her know that you were on her side, and not your brother's, and would be willing to help her with anything that she might need in the future.

Not into the idea of screwing around with your sister-in-law, huh? Maybe you should start from the beginning and pick a different thread.