Regulus, it seemed, was not a boy who had learnt yet how to hold his drink, at least by the sound of the giggles that erupted from him. If someone didn't catch hold of him now, Lucius had no doubts the boy would end up with his face in the mud, and probably still giggling. So instead he moved, catching one arm about Regulus' waist and holding him, turning him back to the others and murmuring, more to himself than Regulus, "I know girls that can't giggle like that."
He looked sharply up at Rodolphus then, having caught the last few words. "I'm glad you are so protective. Rodolphus, but he's got to do some things for himself. Rabastan is going to sit this one out, is he? While everyone else is going to take part, because they don't have big brothers to stand up for them? If he doesn't want to, let him say so for himself." He was not about to let this become another part of their struggle for power, he was not going to be one-upped because Rodolphus could wrap his baby brother around his little finger. Rabastan was not an extension to him, and Lucius was not going to be undermined because Rodolphus just wanted the captaincy to himself, and was about to try and create discord among them.
And so he pointedly turned his scowl from Rodolphus, softened his expression, to show he wasn't angry with Rabastan, at least, not to the same extent, the boy had done nothing wrong himself but been shy. "The choice is yours, Rabastan. But if you choose to stop, you've got to carry this one up to the Common Room." He said, nodding towards Regulus. "And keep him quiet all the way there."