Until the age of ten, Vayne Solidor had been his brother's world. [Spoilers for the end of the game, written for FF_100's prompt "Rather be lonely in love, than alive with you and dead."]
At ten, though, Larsa's unofficial education in politics via Drace and Gabranth turned into behind the scenes machinations, and his view of his brother was forever altered.
No so altered, however, that he didn't imagine he could sway his brother even at the last, even with the Resistance fleet bearing down on them, even with the Lady Ashe and her companions bearing down on them with weapons raised.
Not so altered that being proved wrong didn't shatter something inside him.
His sword was steady; Gabranth would have been proud. "No. No, brother."
The men of House Solidor were brought up to place others' needs before his own. The need of Dalmasca - their need to have their queen returned - outweighed his own need for his brother, and his brother's need to have him by his side as he ruled.