...for her pleasure.

You fuck Karen's warm snatch for the next ten minutes or so, from missionary, to doggystyle, and then back to missionary with her feet up by your head as you dick her down hard. You have your hand pressed over Karen's mouth. You learned right away that you needed to muffle her, as she's quite loud in the sack.
"Fkkkk...." she moans against your hand as you spear deep into her. "Fkkk....." her eyes roll up, and then squeeze shut as her pussy spasms. It's her second orgasm of the session.
Despite how hot it is to be fucking your wife's sister, and how hot and sexy Karen is, you aren't having success reaching your own peak so soon after nutting with Jenna. You slowly wind it down, kissing her feet and legs, inside her knees and thighs, up her stomach and suckling on her tits for a while as she comes down.
"Do you want me to suck your cock, James. Will that help...?"
"There's nothing that needs help, Karen. I don't need to nut every time. I just wanted to make you feel good..." you smile.
"Wow. You really are just the opposite of Jack in every way..." she strokes your cheek. "I always told Jenna she got the good one. I didn't even know how right I was..."
"We'll have plenty of time for more fun." You give her a big kiss.
"Thank you for this, James. And thank Jenna, too!" She lays back. "I'm going to just give her the biggest kiss in the morning."
"Yeah, about that..."
"What...?" she looks at you with concern.
"We...didn't exactly lay down our ground rules yet, so I don't think she expected me to just...fuck you... right away. So maybe don't... mention this?"
"James..." she looks at you with concern. "That wasn't... I enjoyed this so much but you shouldn't....have.... I feel bad, now."
"It's okay. We're going to hash out the details tomorrow and everything will be fine. Just don't let on that I got a little head start..." you slink out of the bed, pulling on your boxers.
"Oh...ok...." she seems uncertain, but finally nods.
You carefully climb back into bed, but Jenna wakes, groggy.
"Bab...e...?"
"Bathroom..."
"Oh...k.... g'night...sexyy...." she reaches blindly to touch your chest, and you grab her hand, squeezing it.