Keep snapping and teasing.

It'd be very easy for you to "win" by dialing up the invisible knob in your brain you can now feel attached to your sister's attraction level. But, even without you doing so, a few snaps is already enough to have her sneaking glances of your junk. Maybe, just maybe, there's a way to force her to succumb while feeling entirely like it was her idea in the first place. You take another step, now close enough to reach forward and lean your palm against the arm of the couch. She looks down at it for longer than when she glances at your cock, but then makes eye contact again.

"The least? Well, I know that, but--" you snap to punctuate your sentence, and you have to resist the instinctive throb and flex of your cock when you see her pupils dilate and her breath quicken. The look in her eyes is less focused, scattered and torn between her lust and her self-control as you've all but closed the distance between the two of you. "--I just can't seem to think of what the most I could do is, if that's the least."

Another snap as you point at her, and her breaths are coming out heavy and panted now as her hips wiggle and her thighs squirm against each other on the couch. For a moment her attention lapses and she runs a hand up her flat stomach towards her massive chest, but she catches herself only a split-second after she touches her underboob. "So! Got any ideas?"

"Uhm- uh..." Laney swallows, her eyes jumping between your hand on the couch, your massive erection, and your face. She looks a little more cross-eyed when she makes eye contact, worrying her lower lip with enough force you're pretty sure she's about to break the skin. "Well- um- help me with one of my photoshoots and videos or- or something. I could have a dynamic camera if you're holding it."

"You mean your thirst-trap posts?" You ask incredulously, almost grinning when realization crosses Laney's face that you've somehow gotten her in a corner. "You want your brother to record you dressed in all sorts of skimpy or skintight outfits?"

'She's what now, a six on the scale?' You wonder, watching smugly as her jaw opens and closes with a clear struggle to come up even with an excuse.