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The Vilna Gaon (OC 470, s.v. v'ein) explains the opinion that exempts women from fasting as due to the fact that women lack k'dushas b'choros (the sanctified status of firstborns). A male with an older sister also lacks k'dushas b'chor since he is not a firstborn, so the opinion that exempts the older sister would certainly exempt the brother. In fact, a ...
The firstborn, if a son, fasts. If the firstborn is a girl, then the first son is not the first born for the sake of this fast. http://sephardicjudaism.blogspot.com/2007/03/passover-first-born-if-son-fasts-on.html
You can listen to him discuss it here at 56:12. He says it isn't rau'i l'achilat adam (fit for human consumption) so it isn't considered eating. He quotes some opinions that it is a problem of washing the inside of the mouth on Yom Kippur, but he doesn't seem so impressed by the idea.
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