Grab her tits

You get right up to the tub, admiring her ass before you make your move.

With one hand you undo your pants and start jerking off, before reaching around and grabbing her right breast roughly. It's soft yet has a nice firmness to it, and is still slightly soapy from her washing herself.

She shrieks before turning around, though you don't let go.

"What the fuck are you doing you pervert? I'm your sister!" She yells at you before pushing your hand away.