Going for the bottom first

Desire burned in Emma's mind as she explored the firm flesh of her younger sister, and her new cock kept twitching in anticipation. She knew she couldn't wait much longer, or her head(s) would practically explode with frustration, but still she hesitated as she worried that her sister may wake up if she couldn't control herself.

She had to do something at least, if just to keep her sated for a while, and so she rolled Hannah over on the bed to get a good look at her in another position. And as soon as she saw that panty-clad little bottom, she knew exactly what she'd have to do.

Just a moment later she'd grabbed one of her sister's pillows and put it under Hannah's slender waist, propping her up so that her firm ass became the highest point of her body, while also pulling her back to leave her legs hanging off the side of the bed.

Now she had excellent access to those sweet-looking buttocks, and couldn't wait any longer to make use of them. At first she just groped them firmly through the flimsy white panties, as if to check their density, and then moved on to push her own crotch to those lovely hills. The idea and sensation of what she was doing pushed her to another level of lust, and it wasn't long before she'd whipped out her shaft and started stroking the head of it against the smooth white cotton of her sister's panties.

"Oh, I can't take it!" She heard herself groaning, as if through a haze of desire, and filled her palms with Hannah's buttocks while starting a steady dry-humping of the cleft between them. She watched with delight as a small amount of precum dribbled from the tip of the shaft, forming a growing trail along the crevace of her sister's ass, and then pulled away for a moment to enjoy even more the sight of another kind of wetness forming a little further down. Even in her deep slumber, it seemed like the girl was getting just a little turned on by the action...

"Yeah, that's it...!" Emma grinned down at her sister, though she obviously couldn't hear her at the moment, and then let her cockhead slip down to brush firmly against this growing wet spot. The sensation was truly exquisitive, and Emma kept trying several different angles to make the best out of it, either pushing herself down between her sister's thighs, or returning upwards to spread that sticky wetness throughout the valley between her silky buttocks.

Finally she couldn't last much longer however, and settled for a steady humping of Hannah's buttocks, until suddenly exploding all over her back. Jet after jet of thick white cum went flying through the air, splattering upon her sister's milky skin, before Emma managed to regain just a bit of control of herself and somewhat try and limit the area of effect. This resulted in Hannah's panties becoming completely soaked through however, as the last couple of spurts ended up directly in between the tender globes of her ass.

It took Emma another couple of moments to come down from her peak, and then survey the damage done, and she found the results to be strangely enticing. Her own juices where now mingled with her sister's, all over both the front and back of her underwear, and somehow the sight was making her cock twitch and heave once more while slowly starting to regain it's full posture.

Now she couldn't help but wonder what would've happened, and what it'd be like, if there weren't any panties in the way to begin with...