Investigate Heather's room.

You tiptoe over to your sister's bedroom door. Listening intently, you hear mumbling and low moans. Your curiosity is peaked, so you gently, oh so quietly, turn the knob on the door. You slowly open the door just a crack. The room is dimly lit. Your eyes widen at the sight presented to you. You see a tangle of limbs and bodies on Heather's bed. Your sister is on top of someone else that you can't quite make out, since Heather is kissing them and her hair is tumbled around the other's head. She breaks the kiss and lifts up her head slightly and the other's face is revealed. You are shocked to recognize Trish beneath her. You never knew that either of them was interested in women.

Their hands are rubbing and pinching each other's breasts. Each has a leg between the other's legs, and they are grinding their pussies into the other's leg. Trish moans softly, and pulls Heather's head back down to hers, kissing her hungrily. They kiss wetly for several minutes, and then Heather starts kissing down Trish's neck, flicking her tongue at the hollow between neck and shoulder. Trish shudders and moans again. The trail of kisses leads down Trish's chest to her right tit, which Heather circles teasingly, kissing with occasional licks of her pink tongue.

You are very turned on by this girl-on-girl action. You open your pants and start rubbing your cock while you watch.

Heather's tongue licks lightly across Trish's hard nipple. Trish is writhing and groaning under your sister's passionate attention. Her hands are stroking and pulling Heather's hair, and caressing up and down her body. She arches her back, pushing her nipple into Heather's mouth, who sucks on it noisily. "Mmmmm, that feels soooo wonderful. Don't stop," pleads Trish.

"Mmmmm," is all Heather says, but the vibrations obviously excite Trish, whose leg bumps harder into Heather's sex. Heather's tongue glides between the tender hills of Trish's chest and back up the other side. Her tongue circles around the swollen nipple she finds there. She sucks the nipple into her mouth, and Trish gasps louder. Trish groans repetitively and her hips start pushing up hard into Heather. "Cum... oh ... Cumming ... I'm ... oohh oh oh oh ... cumming," she whispers.

Your cock is slick with pre-cum, as you stroke it. You can't believe you're watching your sister fuck Trish. Your supply of masturbation fantasies just got refilled to the top.