His brother made a grievous mistake in remaining inside of his smokescreen. While his lightning arrows had been focused in the middle, he had made sure to attack all throughout the cloud. If even one hit, no matter where it hit, it'd prove painful as all Raiton techniques do. Though his next action determined whether or not the smoke contained chakra. If it did, he'd be hard pressed to see through it with his Sharingan. If it didn't, he'd be able to see his brother's chakra inside with little problem. Though, he wouldn't wait too long to give his brother time to recover from his near blindness.
Though if his brother still decided to continue with his blue vapor attack(if hit), once they were visible through the top of the smoke cloud, he would have instinctually grew active. Unsealing a physical arrow as he pulled on the string of his bow, aiming at the bubbles of blue vapor and releasing the arrow in quick succession. He rinsed and repeated the process mentioned above before they could rush towards him. He was a master archer, hitting the baseball sized orbs of smoke would take little to no effort. The popping of the smoke bubbles, would cause the neurotoxin inside to release around the immune Iburi.
Continuing with his assault of the arrows, he'd unseal another arrow and pulse it with wind chakra. He rapidly release it towards the cloud of smoke. As soon as it was ten meters inside, he'd release the technique inside of it. At his command, the wind would burst forth in tiny blades showing that it was his External Retreat technique. The release of this also depending on where his brother was, obviously intending on releasing it as close to his brother as possible.
Though, if his brother had been hit and taken a different route, whatever that may be, he would do nearly the same thing. Unsealing two arrows at once, he'd aim them towards the cloud (if he could see his brother's chakra, then he'd aim it squarely at him) and pulse his chakra into them as he released them. A full meter from their quick release, they would rapidly multiply from two to one thousand(1), toning down the technique a bit. Taking advantage of his brother's temporary blindness, the chakraless arrows would close the fifty meter distance between them in less than a second, a testament to the sheer speed arrows can fly from a compound bow like Goliath (300 feet per second) all his arrows having this level of speed.
Of the thousand arrows, and because of the tight grouping of the arrows, two dozen had the chance of hitting his brother. It was a wall of arrows that would be more wide them tall, though it'd leave a single meter on both sides (left and right, from where he was) completely arrow free, completely covering the top.
Regardless of whether his brother chose the blue pill or the red pill, at the end, he'd pulse his chakra and his form would begin to vanish from sight(2) to the point that the Byakugan wouldn't even be able to see his chakra. He would have to physically feel him out. Though, the downside of it was, once he releases an arrow, or any attack, they would become visible. Which meant, he would have to use his speed and stay moving. He would also have to implement his grand plan whilst like this. If his brother became a pile of diced blood and flesh or a literal pincushion, he would know eventually.
1- [Yajirushi Kage Bunshin no Jutsu/Arrow Shadow Clone Technique]
2- [Suiton: Mujin Meisai/Water Release: Dustless Bewildering Cover]