Incest Seduction/Ignore it and keep watching TV
It's all innocent enough, right? Her hand isn't doing anything. She's just getting cuddly. It's not her fault you went commando today. And so what if you have a semi?
You know you're just trying to justify all of this to yourself. You're riding a dangerous edge. But acknowledging that you like this is a little too much for your conscience right now. You decide to hold back your uncertainty in order to enjoy this moment with your little sis.
"Why did you stop?"
Her voice is startling. You could swear you didn't stop petting her, but it seems you've been stuck inside your own head for minutes now.
"Oh, sorry sis. Guess I got distracted."
Kim giggles into your lap, sending a few more vibrations to your cock. You resume stroking her hair and her skin but not even a minute has passed before she has a new request.
"Hey Zaaack..." She practically moans your name. "Could you rub my back?"
Your cock twitches again. "Uh, yeah, sure."
It's a little awkward with the way she's laying on you, but you move a hand to her back and gently press your palm into the fabric of her shirt.
"Nuh-uh. That feels weird. Don't rub my shirt, rub my back!"
You scoff audibly at her seemingly ridiculous complaint. "Wow, sorry princess!"
"Fine." Kim sighs as if you've now bestowed an incredible burden on her. She sits up from your lap just far enough for her to wiggle her way out of her tank top. Well, 95% of the way. She doesn't slip the top over her head, instead letting the garment rest around her neck freeing her arms and her torso completely.
"There!" She lowers herself back down onto your lap, her palm finding the exact same position resting over the crotch of your sweats atop your shaft. She rotates her body away from you, if only slightly, to give you more access to her back.
You can't help but inspect her bra. The pink lace is teasingly transparent with tiny opaque flowers and leaves dotting the material making it difficult to see through. You can't help but wonder what it would look like from the front. And then you wonder why anyone would wear this kind of underwear with a yoga outfit. Maybe it's a fashion thing?
Your cock is far too happy right now. This is still your little sister you're ogling, but he doesn't seem to know any different. Or maybe worse, he knows what's going on more than you do. This is just a back rub, right? Get your mind out of the gutter!
Slowly you bring your hands to bear against her back. Long gentle strokes against her bare skin easily coax a few pleased groans from your sister.
"That's nice." Kim repeatedly shifts her head, in turn rolling her small hand against the tip of your cock ever so slightly. For a brief moment you think about ending this. You can't afford to rise to the occasion any more than you already have. But then she stops squirming, perhaps finally making herself comfortable.
Nothing wrong with a back rub you reassure yourself.
You navigate your hands around her narrow back, taking care to avoid the straps of her bra as much as possible. Your fingers don't need to remind you of how little she is now wearing. But after a while it becomes apparent that you're avoiding the straps a little too purposefully.
Kim rolls her back into you and turns her head upwards as she asks, "Is my bra in the way?"
You can't stop yourself from glancing down at her modest cleavage while she stares up at you in confusion.
"Oh, no!" you deny a little too emphatically, almost sarcastically.
"Are you sure? You can unhook me if you need to..."
Might as well unhook her bra. For the back rub, of course.
You're having a hard enough time as is. No need to make it worse.