It's been a month.
"You have the tendency to put your weight on your legs when you shoot." As Yuuko lectured Yukio, Rin stared nervously at the various batting machines pointed at his bespectacled brother. Each ball can pack a mean punch if it hit you. "You have to be comfortable with having no grounds to absorb the kinetic energy of the bullets propelled."
Yukio had his brows furrowed so deep he looked constipated. "Understood."
Rin and Shura had had Yukio experiment with a number of exercises to improve his control of the blue flames. They tried the candles first, of course, but Yukio seemed to be a different mold of demon from Rin, because rather than burning down all the candles, he had difficulty even lighting up one.
"Nii-san!" Yukio had snapped at him, "Don't shoulder more of my powers in you! That's counter-productive!"
Rin flailed his arms. "I'm not, I swear!"
Shura and Yuuko had their faces folded. "Fire doesn't seem to be Yukio-chan's domain…" the petite woman thought out loud.
"Ah, so troublesome," Shura sighed. "Bibiri, let out all yer powers now!"
"Oh, I see," Yuuko exclaimed, then nodded along. "We'll only find out after you let out enough of them for us to see, Yukio-chan," she explained gently.
Yukio looked really unsure. "But-"
"We'll catch you," Yuuko placed a reassuring hand on the tall male's broad shoulder, "you can trust us."
"…Yuuko-san." Rin couldn't stand to hear how moved his younger brother sounded.
Shura clucked her tongue. "Such a scaredy cat!"
"Shura-san!" The knight was glared at intensely. "I've decided to not have expectations of you long ago!"
Eventually, Yukio did released more of his powers, and brought a typhoon into the room.
And that was how they discovered Yukio was an air elemental, with tendencies to defend or enhance himself rather than attack.
"Don't shoot anything but the golf balls," Yuuko's reminder brought Rin out of his memories, into the batting centre that had been rearranged to be Yukio's butchering table, if anything went wrong. "Also," she added belatedly, "take off your clothes."
"Hah?" Yukio's eyes looked like they were going to pop out of his socket. Rin too, jumped from his perch on the side of the room. "What for?"
"I know you're a perve, but…" Shura said disdainfully.
"No, idiots!" Yuuko shook her head. 'That's a bonus, but…' She murmured, not seeming flustered in the least. "All those potions, magazines and canisters of holy water, not to mention the thick overcoat, are weights dragging you down and serve as unnecessary wind resistance. You need to get used to them gradually, but not now. Besides," she showed everyone a smug expression, "How many shirts have you torn giving your wings a material form?"
Yukio pinched the bridge of his nose. "…Understood."
The nearly six foot young man took everything off, leaving just his underwear and shorts (and of course, glasses).
The sight caused desire to pool in Rin's gut. Even though he'd never considered himself as anything but straight, and the moley four-eye's body was so male. All lean muscles on his legs and larger biceps from wielding high-powered guns. No wasted softness or curves.
Yuuko openly ogled her student (what a pedophile!) before walking over to the system panel. "One magazine is enough, since there are only 6 golf balls," she poised her finger on the button. "Ready?"
Yukio gave a sharp nod, anxiety in every line of his body.
"Special Infinity mode, selected."
Rin didn't dare to blink. Having one batting machine constantly firing balls at you at such speed was scary, having several must be downright suicidal. He could see that Yukio was struggling right from the start.
"Keep your body streamlined," Yuuko coached, "and release more power! You can achieve a higher speed than this!"
Yukio frowned as he dodged the balls heading for his sternum and right knee, and fired his first shot.
A golf ball dropped to the floor and rolled away.
"Good! Keep it up!"
Yukio looked pleased with the praise, but half a second after that, six balls charged straight at his body, and he…lost control.
The blue flames around his body burned bright, his wings twisting as they brought him higher to the ceiling. Simultaneously, an erratic barrier of wind began to form.
Rin could sense his bondmate's panic through their connection. "Yukio!"
Then, in the blink of an eye, the blue flames vanished, along with the wings. Yukio dropped seven feet to the ground.
"Yukio!" Rin quickly launched himself to his brother's side. "Are you all right?"
"…m…fine," the moley grunted as he rubbed his tailbone. All right, my arse.
"Typical of Bibiri, to run away the moment yew feel insecure," Shura sighed. "Unlike yer guns, this power doesn't come with safety and triggers." 
A vein pulsed on Yukio's temple. "One more time," he told Yuuko and spread his wings, his eyes burning blue.
"Yukio!" Argh, why was he so stubborn!? Before the taller male took off again, Rin grabbed his wrist and shared something he'd learned from the candles exercise, "It helps to envision the wind in shapes you want them to take. Ah, and picture their detailed position, too!"
Yukio stared at him in surprise, speechless for a moment, before he smiled. "Thanks, Nii-san."
Rin found his cheeks growing warm.
Don't say it like that when you're sweaty and half naked, idiot!
"Special Infinity mode, restart," the system announced.
Yukio appeared calmer as he faced his 'opponents'. He flipped around on the air to dodge the three balls aiming for the back of his head, and fired another shot.
"Picture a half blade under your wings, supporting them," Rin shouted his suggestion, and was giddy when it was taken up, to Yukio's advantage.
"A-aand that's the sixth," after fifteen minutes, the exercise came to a close. "Result's good, for a first exercise," Yuuko looked content.
"Thank you," Yukio replied his teacher with a muted pride. He turned to Rin and hesitated, before saying, "Controlling the flame is…harder than I thought. I'm sorry I…made light of it in the past during the candle exercises, Nii-san," his determined, self-conscious face as Yukio mustered his courage to tell Rin these made the elder male's heart falter.
And Rin's eyes couldn't seem to stop following the trail of a bead of sweat as it slid down Yukio's skin, from his neck to his chest to the ripples of his abs.
Yuuko must have noticed something, because she cleared her throat and pulled the taller woman by the hand. "Ah, I forgot we have an Upper First Class meeting! Yukio-chan, debrief you later, okay?"
Shura appeared surprised. "Hey, we don't-"
But the petite woman had managed to yank her colleague out of the room with her. "Bye!"
As soon as they were left alone, Rin followed his libido and forced himself into his younger brother's personal space.
"Nii-san?" Yukio's eyes widened when Rin laid his hands on his cheeks.
"You have two seconds," Rin told him, his breath seeming hot. "1, 2," he crashed his lips onto the other male.
Rin didn't know how he could have waited one month. So good, he thought as he pressed against the solid wall of muscles that was Yukio's chest, his hands roaming down the taller male's back and mussing his wind tousled hair. So right, the closeness felt like coming home. Rin wanted to bury himself under Yukio's skin and stay there.
"Ah," Yukio moaned and pressed back against Rin for a moment, before he struggled and pulled away from Rin's face. "Nii-san!" his face was bright red, his spectacles askew and foggy from heated breaths. "What are you doing?"
"What am I…" Rin leaned forward in a daze, his face searching blindly for his soulmate's before he sighed when he realized Yukio wasn't going to give in without any explanations. Gee, the one time he actually wanted to talk… "We are bondmates. Romantic bondmates. What's wrong with being intimate with your own bondmates?"
Yukio blinked, the blush on his cheeks dissipating. Rin missed them already. "Ah, I thought…" the four eyes attempted to retrieve the use of his arms from Rin's tight embrace, but Rin wouldn't give it back. "You...initiated the consummation because you wanted to help me control my powers…"
"Well…" Rin threw his gaze. "There is that…but…" okay, it's Rin's turn to confess. The elder twin inhaled deeply and faced Yukio head on. "I want to experience this soulmate business with you. I want to grow older with the person who's meant for me, the person I'm meant to be with. I…didn't want to lose you." Still, Rin couldn't help but droop his head after he finished.
What a selfish reason.
"But…" Yukio didn't sound bothered or disgusted. His voice was still predominantly filled with shock. "Aren't you straight, Nii-san?"
Rin swung from guilty to irate in 0.1 second. "So what?" He snapped at his companion, before his eyes widened in disbelief. "You don't seriously think I will still…chase girls after..."
Yukio's lowered gaze was enough of an answer.
Rin snarled, before snapping his hips upwards, rubbing his hardness against the moley four eye's hips. This epitome of a Japanese teenage male. Yukio gasped in surprise.
"How? Do I still seem like I don't want to fuck the living daylights out of you?" Rin reverted to his delinquent language in his frustration as he continued rocking against his companion. "Would I be like this if I didn't find you attractive?"
"N-nii-san," Yukio panted, and started rutting back, the bulge in his groin growing harder against Rin's dick.
Rin very much wanted to stick his hands into this 'I-always-think-too-much' guy's pants and start stroking (so he could see that collected façade break completely in pleasure), but he needed to say this.
"I want to experience this soulbond with you," Rin repeated the words to Yukio's lips, earnest blue eyes staring straight into the other end of his string. "Every single aspect," he ended with a kiss.
(Rin should have known better than to expect that had solved everything. Word is cheap.
But at least, he has a lifetime to prove it.)
1. I guessed this is why Yukio the control freak chose to be a Dragoon as his offensive Meister, rather then a Knight. Most powerful knights made a contract with a familiar to enhance their Koumaken (Arthur with the spirit within Excalibur, Shura with what I suspect is Yamata no Orochi, the eight-headed serpent, since her spell is 'Devour the eight Princesses, slay the serpent', and even Rin with Ucchusma in the Kyoto arc). It's not like Yukio has no talent as a tamer at all (the Naiads), but there is always a risk of your familiars disobeying you when your heart wavers. Guns, with safeties, triggers and other mechanisms are 'easy' to control.