Nobody complained when Lyn, a 15-year old, was.
https://www.reddit.com/r/FireEmblem...t_remember_the_outrage_when_is_put_a_15_tear/
It's just more noticeable when one actually looks very young and the other doesn't, not to mention the Age change when it came over to the West, where Lyn became 18/19. Nevertheless, the double standards are still there.
Is it really a double standard though if western audiences were only exposed to the localized age though? The thing is for the majority of people who played her game/are familiar with the character, they are only going to know about the age change if they dig up that detail since it's not exactly important knowledge that is commonly shared. Additionally, Lyn doesn't look or act like a young girl in any of her portrayals (whether English, Japanese, or through her different alts and game appearances), so even if you're unfamiliar with the age her appearance and characterization would still have you believe she's an adult or at least close to it.
With Sanaki, not only is she introduced as a child in PoR (where she's first introduced to fans of the Tellius Saga) and a young Teen in RD, she's also portrayed as pretty young in Heroes as some of her quotes have her act a bit childish such as asking for candy. So there's a difference between Lyn and Sanaki's portrayal right there that may stir up more concerns with the latter then the former. Additionally, the artwork does lean more on her younger appearance from PoR rather then her RD one in her base form, which leads to some other questionable implications that Lyn never had.
In short, the 'double standard' only exists for those aware of the localization change, and even then ignores some of the finer details behind the outrage around Sanaki. Lyn is never portrayed as a young girl to audiences to stir up any concerns with being a bride, whereas Sanaki has and is still portrayed as young or at least younger then the other brides around her.
With all that said, it seems the localization noticed this beforehand as there were already a couple changes with her. Her original title was "Bridal Apostle" rather then the more ambiguous 'Apostle in White', and her spoken line in the trailer was more tsundere in the Japanese version compared to the more blunt tone the English one has.