FG: I do not know what happens, if anything happens, when I find my big brother. I'm guessing the unique flavor/role name is due to me being kidnapped all the time? I don't know Seto's role either, so I don't know if he has a similar ability or not.
When I say sibling-esque, I mean your claim of Joey being a "VT Sibling" with Serenity. Seems odd to have two sets of roles that interact like this.
In addition to Seto possibly being a PR like you guys are discussing, it's also possible that Seto is a scum role or third-party, because my PM said I don't know his alignment either. In that case, my big brother most likely won't claim, as scum-Seto nameclaiming would be too risky with one of the mafia down (since I'm 99.8% sure there's either only one or two mafia left at this point) and TP-Seto would create similar issues for that person's wincon as well. This is a likely scenario IMO,
IF
FG is town-Joey, as I can't see two pairs of "siblings" as town in the same game. /speculation