Robin wanders in...

You press yourself against your mother, sighing and blushing in a delightful counterpoise to your flushed and eager excitement. Your hands knead into her soft, natural breasts, thumbs gliding over her nipples and feeling them harden. Heather sighs and rocks against you, her eyes opening up to look at you with a startling expression that seems to say: this better be real. You moan and start to pinch over her nipples, her hand reflexively tugging on your erection with a delightful roughness.

"Oh come on, Michael--we don't have all day!" Robin's voice chimes in from behind you. It's not with a tone of utter scandal, maybe a little bit of surprise, but more about peevish sibling annoyance. Her brown eyes twinkle with a little mischief as she sits on the edge of the bed beside you and Heather. "I mean, I get it, mom's hot and dad's out. And it's flattering that I got you this worked up, but seriously! You have to get a girlfriend."

Robin is still wearing her blue bikini--and while she's not blessed with your mom's curves, your sister is lean and teasingly cute in her own way. "So how far--whoa, mom!" she says with genuine surprise as she sees her hand down the front of your pants. You're pretty much frozen in place at this moment, but fuck all if you don't find a way to get even harder. The look in Robin's eyes reminds you of the time she caught you daydreaming in her general direction at a ice cream parlor. It was weird, and you didn't understand her smirk and wink at the time, but now the wheels line up in your lizard brain. "Did I walk in on something naughty?" she giggles and leans over you, her long brown hair brushing over your cheek.

"As a matter of fact, young lady," Heather says, shyly pulling her hand out of your pants, reluctantly releasing her grip on your cock. "I was just telling Michael he needed to find a nice girl his own age."

"Yeah, it looks just like that," Robin says with a sarcastic tone and a vigorous nod that tosses her sweet-scented hair over your face. "Mikey? Is that the story you're sticking with?"

"Um, yeah? I mean it's not like we were going to do anything," you lie terribly, just then belatedly thinking that the Rulebook could have prevented this sort of interruption. "I'm just working off steam, you know?"

"Oh, believe me I know," Robin says with a beaming smile and a heavy sigh. She scoots back on the bed to get a better view of the two of you, bumping into the Rulebook you'd sort of just tossed to one side when Heather started to open up to you. "What's this: some kind of journal?"