Oh, boy. What a fun thread this has become!
I guess very few people understand, if at all, that it's the principle of cutting into my personal time that gets to me. Like I said, the other bathroom is detached from the house, and I see no reason why I would have to make the arrangement of using that one when I'm supposed to be using the main bathroom during my scheduled time. Yes, I can be immature and uncompromising about it if I want to.
Anyway, this issue has been resolved as my brother realized the error of his ways and promised not to interfere with my personal shower time. Case closed.