If she's offering, you're game

To answer her, you grab her slick body and pull her next to you to kiss her. She kisses back, using her tongue to explore your mouth. Her hand caresses your shaft as you make out. After a couple minutes of kissing, you say, "It was a little weird when I thought I was fucking Trish, but we should have done this long ago."

"It almost happened that time we all went camping in the mountains," Heather says.

"Really?" you ask.

"Yes, remember the bear coming through to find food at night? I almost went to your tent for protection. I think we would have then."

"If you'd come into the tent all scared from the bear, you're right. I think I would have comforted you, and I probably started touching you to distract you."

"You mean like this?" she says as she takes your hands and puts them on her boobs. You squeeze them firmly and then pinch the nipples. Your sister moans and leans up to kiss you again. You kiss her hard, thrusting your tongue deep into her mouth. Then you turn her and pin her against the shower wall. Heather pushes her ass back toward you, looking over her shoulder with a fiery gaze. You reach around her, cupping her tits and rolling her nipples in your fingers.

"Ooooo, that feels good!" she says with a purr.

After some good fondling, you take one hand and guide your cock between her legs. Her hand takes the end and slips it into her hot, wet slit. You grab her hips and thrust forward, just as she pushes back. Your cock slices right straight through your sister, and you bottom out, growling in her ear. She shudders and moans as you fully impale her cunt, so wet and hot.