Use her ass, but someone interrupts

"You always did know more about medicine and stuff," you say to your mother, stroking yourself back to hardness. "Of course the cream should go inside... Duh! Bend over a 'lil more, would you?"

"Right now? Um, okay, like this--Oh!" The moment she leans forward, you press your cockhead against her asshole. "...It's warm?"

"Mmm... Mmmhm, I ran the bottle under the water to make it more comfortable," you reply, hoping she won't realize that she's standing between you and the shower. "Huh, this bottle's pretty big... Would you mind grabbing your 'rear' and, I dunno, spreading it a little wider?"

From what you can see over your mom's shoulder, she's flushing with embarrassment. She obeys without question, however, pulling her ass cheeks apart as you work your way further inside.

"Uungh..."

"A-are you okay, S-Sweetheart?"

"Ah... Huh? Oh, yeah, fine. I'm just having some trouble pushing this in," you grunt. "Here, lemme try something..."

You reach out to take hold of her plump ass, then stop short as you realize something: If I use both hands, she'll wonder how I'm holding on to the "bottle."

That thought is quickly followed by another: Who am I kidding? She isn't NEARLY smart enough to notice stuff like that... Not anymore.

Grinning silently, you grab your mother's voluptuous body and begin humping her like a man possessed.

"Ah... Aah! Um, Marcus," she yelps. "Why are you wiggling around like that?"

"J-just making sure I squeeze out every... Last... Drop," you reply, punctuating with a few good, hard thrusts into her asshole.

Just then, you hear footsteps coming down the hallway outside. Please be Cassidy, please be Cassidy... Fuck, I didn't even lock the door, you think, fucking Mom more slowly as you listen for the tell-tale sound of your brother's--

The door bangs open. You can't see who it is through the heavy shower curtain, but you're hopeful; after all, Cassidy has a habit of interrupting your one-on-one time with Mom, and he loves storming in on her in the shower, so--

"Jenna? Oh, I'm sorry, I didn't know you were still in here."

Fuck... It's Dad.

"He can't find out I'm here," you hiss into your mother's ear, still balls-deep in her ass all the while. "He's gonna think I was using you, and then I won't be allowed to give you medicine for even longer!"

Your mother scoffs. "That's ridiculous, he won't--"

She stops as you pinch her butt firmly. "Trust me, Mommy. He's jealous... Not thinking straight."

"Jenna? Hello?"

"Um... Hi," your mother says meekly. "Don't come in, I... I..."

Noticing that her dumbed-down mind is having trouble thinking of an excuse, you whisper one in her ear.

"...I don't want you to see me like this. I'm scrubbing Marcus and Cassidy's... Medicine off, and there's still lots in my hair, and--"

"Okay, okay, I get the picture," your father groans, clearly disgusted. "Those goddamn kids..."

"They're just trying to h-help, Dear. In fact, Marcus found this new--" You pinch her again. "Ah, um...We can talk about it later. Are you going out?"

"I think I should, yeah... Maybe I'll take a few hours, get my head on straight." Ah, yes; your father's secret code for "fuck a bunch of strangers who don't know that I can't produce semen."

You lean forward and whisper in Mom's ear again, enjoying the way her nervous body is clenching around your shaft.

"H-hah... Honey, can I let the boys give me some medicine while you're gone? Just a little bit?"

Your father huffs, then goes silent. For a moment you're worried that he'll say no, or storm out without answering... But when he finally speaks, his voice is sad and resigned. "Well, you have been acting, um... Different... So I guess you need it. Just try not to let them walk all over you, okay? They shouldn't be fucking you up and down the damn house. Keep it reasonable."

"Thank you, Baby. I really ap... Appreciaah..." She trails off as you pick up the pace, slam all the way into her, then begin to unload. As you pump Mom's insides with jizz yet again, Dad misinterprets the situation.

"It's pronounced 'appreciate,' Dear, but you might be better off sticking to small words," he says, a little bit condescendingly. "I'll see you later."

"Buh... Bye," Mom manages to reply as she braces herself against the shower wall, weathering your onslaught. Luckily, Dad slams the door before he can hear the thud, thud, thud of her breasts against the tiles.

Once the last rope of cum has been squeezed into her tight hole, you take a step back and turn around so that she can't see your dripping hard-on. "...All done, Mom."

"G-good. Thanks, Baby," she replies, struggling to stand up straight. "I think you got it really, REALLY deep... I hope it works well."

"Me too," you lie, watching your two loads drip down her ass. "We'll have to test it out, huh? Give you another anal dose, and see if this one makes you extra smart?"

"Um, right, I guess... No, wait a minute... You heard your father. We're going to keep things reasonable."

"Hmm, what did he say, again? That we shouldn't fuck you 'up and down the house?' Maybe there's a way around that?"

"What do you--"

"I know!" You snap your fingers, giving Mom a winning smile. "We'll just go out on a field trip. He can't complain if we're not in the house!"