I think they've been pushing us to believe blonde is a guy with how outgoing and rambunctious they are which is very unladylike in Japanese culture. One of the themes of this story seems to be bucking norms, so it's not far to assume the sister is just really outgoing. Rather I think the demure brunette seems like the more likely bet for being the brother. In fact, if the laws of manga shenanigans are anything to go on I'm willing to bet our genius of a protagonist actually proposed to the boy perhaps under some shenanigans where he dressed up as his sister that day or something, and his agreement to getting married was his genuine feelings.
Brother may also actually be interested in the boy and the sister may have set this up to give them a fair shot, whether or not the protagonist could actually go through with it or not obviously.