If Gokudera had left angry, Riku would have been a little worried. But his little brother had left upset and confused, and so Riku was less worried about him doing something stupid and more concerned that he might smoke himself into a stupor.
Riku moved his arm around Sora's waist, because that's where it was most comfortable, and lay his cheek against the top of Sora's head, breathing in deeply. His dad called from the kitchen, saying that they'd better clean up their lunch mess before his mom got home, and he was making burritos, did they want any?
Riku called back yes, and kind of slipped his fingers back up Sora's shirt. Because his dad was busy, and he could.