Maybe; try making a deal

You squirm and flinch while your fiendish sister's fingertips knead, compress and grope your balls with dangerous intensity. You think about fighting only to realize you don't feel an urge to hit her- rather, her groping your genitals has you aroused: your dick hardens quickly to a thick mast swaying into her wrists and your heartbeat is pounding fast.

She eyes your erection with momentary surprise, but can't take her eyes off your fat balls: seeing she's too focused, you grab both her wrists and press your thumbs against her tendons so she can't squeeze. To your own surprise, it works, and the pressure on your nuts slowly eases...

Your sister is scowling and struggling, though, so you can't really relax. "Let go! I want your balls!" She growls with gritted teeth. Her fingers try to curl- but can't, not past the tension your thumbs press on her tendons. Even so, this is a stalemate, and eventually she'll get your balls if she is really this obsessed...

You decide you may as well get something out of it. "If you want my balls, I want something from you too!" You demand at her. You feel her grip soften only slightly, then she eyes you with a perplexed but piqued expression and a tilt of her head.

She doesn't fathom what you could want, nor that you're willing to trade your balls... "Like- what?" Your sister asks in the momentary peace.