You take advantage of Mom's obliviousness

"I still don't think I need you here," Mom says as you follow her into the upstairs bathroom, closing the door behind you. "Your father--"

"--Is busy," you continue for her. "Look, if we're cutting back on your 'dosage,' then we're gonna have to keep a closer eye on you." You look her body up and down, staring at her almost-exposed assets in the **** girl outfit. "...A much closer eye... And if Dad's too lazy to take care of you, then I will."

"That's not fair, Marcus. You know he tries."

"Sure, sure. Now c'mon, let's get you out of this," you say, advancing on her and pulling at the thin straps of her "clothing."

"Oh--No, that's--" She sighs as her tits pop free. "...Okay, Sweetie, just be gentle."

"You heard Dad. If this outfit is so 'inappropriate,' then we've gotta get it off you NOW!" With that you rip off her bottoms, throwing it all to the floor to leave her totally naked. "That's better. Why don't you start the shower, while I get you something more suitable to wear afterwards?"

"...Th-thanks." She smiles a little, crossing her arms to protect her modesty... While she barely manages to hide her nipples, she fails to notice that her pussy is still exposed.

"My pleasure," you reply, taking one final look before running off to the bedroom.

By the time you return, the bathroom is filled with steam. Your mother is just stepping into the shower, her back turned as she begins to wash you and your brother's jizz off of her body.

"Forgot the fan," you mumble, flicking the switch. Between the sound of the fan and the shower, Mom fails to hear you stripping naked and beginning to jack off furiously. She remains oblivious even as you stand a foot away from the shower, slap-slap-slapping away at your meat.

The sight of her wet, curvy body gets you off faster than you expected, and soon your balls are churning. Thinking fast, you grab a bottle of shampoo, aiming both it and your cock directly at her bubble butt.

"Here, Mom," you grunt, stepping forward and squeezing the bottle just as you begin to unload on her backside. Your cum and the white shampoo splatter against her skin in equal volume, quickly coating it with a thick layer of goop.

Your mother flinches, but just as she's about to turn around you let go of the shampoo bottle and grab her by the hips. "Woops! Dropped the bottle... Don't move, I wouldn't want you to trip!"

"Okaaay... What bottle, though? What did you put on me?"

"It's, uh... Some kinda new cream," you say, reaching down to grab the bottle. "They say it's good for, um... Anal medicine... Absorption. Yeah."

"Wait, what? You mean for, like, the dose you gave me up my--In there?"

"Yeah, totally. This should help the medicine absorb better and last longer, since we won't be giving you more for a while."

"Oh... Okay, that makes sense." She relaxes, continuing to soap up her body. Just as you breathe a sigh of relief, she continues: "...Buuut... Shouldn't it go inside my--My rear? It feels like you just sorta spurted it all around."

"Uh..." Despite the fact that you just came, your cock quickly twitches to life. "Good point, Mom."