Slap her ass and give it a squeeze.

Your hand feels her tush for a moment before pulling back and slapping it firmly as you grab a handful, giggling mischievously at your mother's reaction. She gasps, her plush lips pursing a bit as she looks down at you with hot cheeks, "Y-young lady! That's no way to treat your mother, what's gotten into you?" Pepsi gotten into you but she doesn't know that, "C'mon I'm just having fun Mom~" You let yourself get a few more handfuls of her thick bum, pulling your bulge up and down against her crotch. Her breath hitches as she looks at you mingling of your privates, "Oh dear… My little girl is so naughty…" You giggle against her chest, letting one hand off her to swiftly unbutton her top, letting her tits heave out, your cheeks burning up at the sight. "Gwyndolin!" You snuggle your face between them as Mom does little more than hold your shoulders with a bit of frustration. "Nn… C'mon Mom I'm just playing, hehe…" She breaths faster, looking away, fighting the urge to moan. With some Swift and aggressive movements, you turn her around, "h-huh?" and bend her over the counter, yanking her waistband down, her plump, healthy ass jiggling out. "Y-young lady!" She looks back at you more embarrassed than mad. You slip your skirt down, sliding your cock in-between her ass cheeks as she pants beneath you. "Aah… Dear, it's so warm and, b-big…" You nod to her, smiling excitedly, "Thank you Mama! You're big too~" You bend over her back, grabbing a handful of her tits, letting your dick start pounding her ass with a lot less lube than she'd appreciate. "A-aah! Gwyndolin! Don't be so- Nn!" Your smile curls into a wicked grin as you pump and fuck her wobbling ass harder, pushing her to begin moaning freely, the scent of sex beginning to fill the room. "Baby… Baby… Aah… D-does Mommy feel good?" Licking up her back, you keep pounding and moaning to yourself, "Nn, yeah… Really… Really good… Aanh~!" Your hips take control, picking up the tempo as you feel the Pepsi filling your dick with vigor, moans getting louder and more frequent. Mom looks back at you, smiling weakly, "G-go ahead darling, haah… You can finish now~" Not a bad idea, you squeeze her ass cheek and pull yourself balls deep, biting her back gently as your dick throbs and flexes inside her ass, your cum filling her up within a moment. The two of you moan and pant together for a bit, before you pull out, your dick drooling cum onto her ass, dripping cum on it's own accord. Maybe you'll have something to look forward to in the mornings from now on.