"Oh, hi." She said it friendly-enough. "I didn't know he had a brother." There was a lot she didn't know about him, actually. He might have three brothers and a sister.
Trying to explain why she wanted to talk to Dean didn't seem like an easy task. But his brother was a good place to start if she wanted to find him. "Well, he sent me a letter. It got me kinda worried so I wanted to come find him. Talk to him." Make sure he isn't going crazy and trying to kill himself. "We haven't talked in a long time, years, actually," Buffy explained slowly. "I was surprised to get his letter. But that's why I -- " She looked around. "Listen, is he here? Cause I'd rather just explain this once. And then you can get all questiony with him."