"In a manner of speaking," Sam agreed. Yes, Dean was here. A twisted and blackened version of him, growing more and more corrupted by the Mark on his arm with each passing day. Sam was actually a little bit surprised he hadn't come for the Blade yet-- but maybe that's where he was, whenever he wasn't home, which seemed to be often. Sam had been over almost every other day this week, at different times, and he had yet to run into his brother. Maybe he was out searching for the Blade, checking places that might be used as lockboxes, wherever else he thought Sam might be hiding it. He wasn't going to find it, though. Sam had been made sure of that.
And he made sure that his thoughts did not stray anywhere near the Blade's location while in River's presence. Instead, he let his thoughts wander down the other natural path available to them, the more emotional one, to dwell for a moment on the feeling of his brother's absence. It was different from not having him here at all, which had been slightly easier to handle, if not necessarily better. That was what the quote meant. When someone was unavailable to you, shut away in another universe, there was only the absence. When they chose to make themselves absent despite being present in the same world, close enough to be available sentimentally even if not by proximity, there were other feelings involved. Insecurities, for one; part of Sam felt he deserved to be left behind, karmically, because of how many times he'd done the same to Dean. But more importantly, something was wrong with his brother and he wasn't able to fix it.
It occurred to him then that his brother and hers weren't really so different, in terms of the roles they usually played. Coming to get them when they were in trouble, watching out for them even when they might be dangerous. Except now his and Dean's roles had reversed, a little bit, and Sam wasn't nearly as good at this as Dean or Simon were.
"I still miss him," he said finally, voice a little rough from the void inside him that he'd allowed to surface for a moment. "You must be missing your brother, too."