Although I posed this question as little more then a fun exercise in speculation I started taking it a little more seriously and thinking through what would change and what wouldn't(I'll be referring to the hypothetical brother as Don).
Let's start with the Key metaphor, this metaphor actually works better with a male character and would take on several additional levels of meaning, and I get the feeling that the ceremony at the end would involve more then a couple of shallow cuts, and could be seen as reversing the Angel/Buffy relationship in season 2(young naive boy sleeps with woman who is several centuries old).
This could seriously complicate Buffy and Riley's breakup. After all Riley is the kind of guy who this kid could look up to and Joyce would probably encourage that, because lets face it Riley's the kind of guy that most mothers want their sons to become. I could almost see a type of hero worship developing and I've got the feeling that Riley comes from a large family and probably has some brothers who would look up to him in this way. OK, fast forward to Riley flying off into the night. I think Don might get pretty resentful about the breakup and would get pretty mad at Buffy. OK, Riley is gone, who is the coolest gut around? I think Don would start mimicking Spike in order to irritate big sis, and you that would drive her nuts.
On a lighter side, instead of Dawn's crush on Xander, we have Don's crush on Willow, Tara and Anya. He gets all tongue tide and stammering (I think Willow esp would get a kick out of it.)
