Even if Tsuna hadn't noticed their brothers, Gokudera did. They were hard to miss. They were RIGHT THERE. "What the FUCK!?" he squawked, his voice going up two octaves.
What!? But when did... and how did... and WHAT!?
It wasn't like Sora and Riku had INTENDED on kissing like that right there in front of the door. They'd just been coming in from the morning of hanging out with each other and had been pushing and shoving to get into the house first and then Riku had gotten pushed up against the wall in Sora's attempt to get in the door, but he'd grabbed Sora and pulled on him and somehow kissing started. Somehow.
Of course, Gokudera was LOUD enough to break the mood. Instantaneously. "Watch your language," Riku managed to get out first before he glared at his little brother... while trying not to blush at being caught.