Yeah, seeing her little brother captivate her boyfriend sounds like a kink most sisters would have. She did like yaoi. She wouldn't mind if you did this, and would likely ask to watch if she knew. The thought comforted you. You still felt weird about having these feelings for a guy, but if a guy looked this much like a girl, was it really that gay? You're the one who's going to be using his penis anyways, not him.
"Okay." You say decisively. "I'll do it."
He wiggles his hips. "Go on! Go on!"
Wow he's impatient. Grunting, you kick off your pants and underwear, releasing your tiny cock. "It's um... Really not very big," you reiterate.
"It's fiiiiiine!" he insists. "Like I said, I'm bigger than almost everyone I know. As long as you have a couple inches you can still hit my prostate."
He was so understanding... You almost felt your eyes stinging with the relief. "Okay..." You part his ass cheeks, revealing the small pink asshole between them. You'd never really considered anal with Zoe, but Brian didn't have another option. You line up your head with his hole and lube it up slightly with your droplets of precum.
"Huh, you're pretty thin," Brian comments. "Oh, sorry. You probably don't want to hear that."
"Thanks," you grumble. He didn't deserve the trouble you put into lubing him up for that comment. Irritated, you shove your hips into his ass with a smack, hilting yourself in him.
He doesn't react, beyond looking back. "That's... Um. No, I won't comment. Just try and find my prostate so it'll feel good."
You see how it is. You're not big enough to make him feel good just by thrusting in. Well you'll make sure he feels it good! It's not size that matters anyways, it's how you swing it, right?! Truth be told, though, you have no idea where the prostate is, so you just start thrusting, hoping you hit it.
"Lower," Brian directs you. He must have a lot of experience if he knows the spot that well. You wonder if he pitches or catches more... Then that thought leads to wondering how it would feel to take his cock. No no no, you're the one doing the fucking here! You can worry about that later. Besides, you still have some masculinity left here.
Focusing on the task at hand, you follow Brian's advice, raising your hips to aim your cock downward. "A little lower. Ah! Yesss, there~" he hisses, followed by a feminine moan. "Oh yeah, keep that up, Zack."
You lean over him and place your hands on either side of him, on the bed. You miss your mark a few times, but start to get the hang of it as you hit it again, then again the time after. In a few more thrusts you're giving his prostate a thorough massage.
"I didn't think anal could feel this good," you moan.
"It's even better going the other way," Brian pants. "Especially with a huge cock like mine."
"Hey!" you protest with an angry thrust.
"Sorr-rrryyyy~" he moans, smiling and burying his face in a pillow. "Ohhh, but you're good. You really know how to swing it."
Finally, something to be proud of! You keep up your pace, thrusting and and out three inches at a time. For the first time since you saw Brian's massive cock, you feel like yours is worth something.
With a cry of pleasure, Brian rises and pushes you onto your butt, sitting his soft ass on your cock. "Let me," he pants, grinding his smooth cheeks against your hips. His hands go to yours and lifts them up, under his top. "Play with my nipples," he urges you, to which you readily accept. He's completely flat, but nipples were nipples just the same, right? You start out clumsily at first, but eventually get the hand of that too.
"Oh god Zack, I'm..." his huge erect cock (it had to be at least four times as long as yours) swayed wildly as he rode your cock like a bucking bronco. "Ahhhh~" with a cry of ecstasy, he stopped. A pulse ran through his cock as it swayed to a stop, then a huge wad of cum spewed from the tip. Seemingly in slow motion, the glob of white liquid shot across the room and struck the door. His fat balls churned, then shot out a second that struck the door right at the base. The third hit the carpet, and in like manner he continued, his last weak spurts running down his cock like waterfall and pooling around his balls.
You were so transfixed on his cock you didn't realize you had your own orgasm. "Did you cum too?"
"Y-yeah," you say breathlessly.
Still keeping you inside him, he turns around, his cock rising to his collar bone. "Here, taste it."
"What? I--" you protest.
"Pride. Ignore it," he reminds you, gripping the base of his shaft firmly and aiming it at you.
Resigning yourself to your fate, you open your jaw as wide as it will go and manage to fit most of the head inside. Your teeth make it impossible to go farther, as his girth is wider than your mouth will open.
"How does it taste?" he asks, clearly enjoying the fact you can't fit it in your mouth.
You light his slit and and can't deny, it isn't unpleasant. It's salty yet creamy. It could work as a sauce on some savory meals. Deeply embarrassed at the position, you remove your mouth from his head and swallow. "It's... good." You admit.
Before he can reply, there's a knock at the door. "Zack?" Oh crap it's Zoe, and she sounds...