This will require some testing.

You spend the rest of the bus ride in a daze. At your stop, you almost stumble stepping down from the bus, then the next thing you know you're in your bedroom. You collapse at your desk, your head in your hands.

"Did that really just happen?"

The events on the bus with Emma keep running through your head over and over. Why did she do that? How did no one notice? She was topless for half the bus ride! She's a little on the shorter side, and you did keep your body turned, so it's conceivable that nobody saw you, but still...

Then your mind drifts back to your dream from the night before, and you remember how Emma did anything you asked of her. She even said "thank you for using me" at the end. Could it be true? Are women - ALL women - free use for you now? Your cock stiffens at the thought of all the hot classmates, teachers, and even porn stars or celebrities that you could fuck. You start to absently stroke yourself through your pants, not thinking it strange to be hard again so soon - you're a red-blooded 18 year-old male, after all. But before you get very far in your fantasy or your masturbation, from across the hall you hear the sound of the shower.

Sofia.

Sofia, your 19-year-old sister, who's taking a "gap year" between high school and college, and thus still living with you and your parents. What that means in practice is that she works part-time and spends most of the rest of her time out with friends or at parties, always coming and going at odd hours. She probably just got home from work, since you saw her this morning and she would have slept in otherwise.

Your imagination, and your erection, are now fully and shamefully focused on the image of Sof in the shower. She's your sister, but that doesn't make you blind. You know Sofia is hot. She's tall, stacked, with big tits that are always prominently displayed in whatever outfit she chooses to wear. Her blonde hair usually falls down past her shoulders, and the rest of her figure could fairly be described as hourglass-shaped. She's had a couple arguments with your mother when she was still in high school, centering around sex and boyfriends, so you're confident she's putting that body to work.

But she's your sister. You don't want to - you shouldn't - you know. Fuck her. But maybe you could ask to see her tits. As a test. You know. For science. Sof would normally never flash you, so if she does, that plus the thing on the bus would be pretty strong proof that your dream wasn't a dream.

You hear the shower shut off. You open your bedroom door and lean against the doorframe waiting for her to leave the bathroom while thinking frantically.

How do I test this? What do I say? It has to be something that gives you plausible deniability in case the test fails. Make it a joke. You should show me your tits, ha ha. Yeah, that'll work.

Your plotting is interrupted by the sound of the bathroom door swinging open. Out walks Sof and your brain grinds to a halt. She is wet from the shower, covered only by a just-barely-big-enough towel wrapped around her body. Her generous tits are bulging out the top, and she's wrapping a second towel around her hair as she steps out into the hallway.

"What are you looking at, John?" That's not unexpected. Sofia has always been cool and popular, and your relationship has, at least in your teen years, swung between ignoring each other and annoying each other. Okay, here we go. Experiment time.

"I don't know, a wardrobe malfunction in progress? You look like you're about to pop out of that towel, Sof." All right. So far so good.

"Whatever, I'm in my own house. It's not like you'd complain, would you?" She smirks and walks past you towards her room.

"I don't think I'd mind. Why, feel like flashing me?"

"Ha," she barks, walking into her room. "No." Well damn, experiment failed. Maybe Emma is just a nymphomaniac. But wait, you phrased it as a question. What if...

"Aw, come on. Show me your tits." You cringe as soon as the words come out of your mouth. They're the words of a perv, not a bratty, teasing younger brother. You can already hear the tirade that's coming.

But it never comes. Now fully in her room, Sofia turns around, rolls her eyes, and pulls her towel down to her waist. Your jaw drops to the floor. It worked. Thank you Jesus, it worked!

Your eyes drink in the sight of your big sister's big tits, her pale pink nipples hardening as you watch. She strikes a pose, then another. She squeezes them from beneath, leans forward and jiggles them for you. Apparently she enjoys male attention, even if it's her brother. "Are you enjoying the view?"

You weren't expecting that. "Uhh, yeah. Definitely." You step forward until you're just outside her door. Gotta keep this going. Act cool. "W-what size are they?"

She smirks again. "They're Ds." She does a little shimmy, her pert young tits swaying only slightly. "And they still pass the pencil test."

"The what?" You tear your eyes from her chest in confusion.

"You don't know the pencil test?" Sofia steps to her desk, grabs a pencil, and turns back towards you. "You lift the boob," demonstrating as she speaks, "put the pencil right under it, let the boob fall..." which she does, though it hardly moves, "then let go of the pencil." It promptly falls to the floor. "If it stays, you fail. It means your boobs are saggy. But mine pass, even though they're big. It means I can get away with not wearing a bra, even in strapless dresses." Well, that's hot. You wonder if Emma would pass the pencil test, though you can't imagine her in the slinky, slutty outfits you've seen Sof leave the house in.

Sofia reaches up to the towel wrapped around her head and your eyes are drawn again to her bare beautiful boobs. "Okay, I want to get dressed, John. Are you done staring?"