Family Ties/Don't warn her and just cum deep in her womb
You decided against warning her and instead just kept going, sucking and biting at her tits while she rode your meat stick. Soon you couldn't hold it anymore, grabbed Heather's hips and slammed her down as far as she could go. You felt the head of your dick penetrated something in her, something that was insanely pleasurable for your big sister as her pussy tighten around you as you both came at the same time.
"AAAHHHH!" Heather screamed in ecstasy. Her back arched which pushed her chest even closer to your face while her arms pulled you tight. You kept cumming in her for a solid 20 seconds before she fell backwards, nearly falling off the edge of your bed in the process.
You admired your seed leaking out of her pussy for a second and then asked, "You alright sis?"
Heather nodded and leaned up on her elbows. "Yeah, I'm way better than alright. That was the best fuck I had in a while and who to think that my baby brother would give it to me?"
You blushed in slight embarrassment but nonetheless smiled at her. She moved up to your side and put an arm around your shoulder to pull you close to her.
"So Timmy, what now? We got the whole weekend to do whatever we want and I do mean," She gave you a sexy look and pressed her chest against your side. "Anything.