....not in a million years, would you have suspected.

"Oh..Oh....Ok.....It's....Ka...Karen...." she moans out almost involuntarily, as if you're fucking the name out of her.
"Who's Karen..." you are confused for a second. You don't know of any of her friends named... Karen.... wait...
"What, your fucking sister?!?!" you almost stop fucking altogether, unable to comprehend what you are hearing.
Jenna looks back, regret and lust mingled on her face. "....yeah... she's told me a few times that I got the good one. Apparently... Jack's not the best...hus...band... he's not giving in the sack, he usually...mm...falls asleep as soon as he gets off...and... he's cheated on her a few times. She... oooof....mmm..." she stops to moan open-mouthed for a second... "fuck! I just came....." she pants for a second, then continues..."shit... three months ago, she caught him getting his dick sucked by one of Allison's friends. I guess they haven't had sex since."
"Wow..."
"Yeah, I feel bad for her. She doesn't want to leave him, but she's not happy." She reaches back to touch your chest. "If...if you're interested... you can...I want her to be happy for once."
"Oh, Jenna. You're such a good wife, and a good sister." You pound her relentlessly until she's just babbling incoherently, before shooting a load into her still-tight vagina.
You lay on the deck chair for a while, kissing and chatting about what this new dynamic might look like, but you both agree to lay down some rules in the morning.
"I love you so much, Jenna."
"I love you, too. No matter what happens, together?" She looks into your eyes.
"Together."

Achievement: Hall Pass! (Convince Jenna to explore an open marriage)