Spencer inverts a cliché like a stage-magician turning a set table upside down and dining from it; one of the underlying assumptions is completely gone, but all the rest are reinforced.
It's basically a Regency romance, this, in which the gently-reared ingenu of a gentry house must Marry Well for their fortunes; when he daringly rescues someone from bandits, and the someone turns out to be royalty pursued by traitors, and moreover the royalty needs to get married too, and they're all beautiful and they like each other... well, we're pretty sure we know how it's going to turn out, and then it does.
But I wasn't mis-spelling 'ingenu' up there; Spencer has a consistent world in which healthy men are so rare that living brothers are the main economic asset of a family. There's a really impressive lack of As-you-know-Bob; the characters give us background while arguing about what to do, but no-one explains all the history everyone knows.
So the funniest thing about the Regency romance is that it makes, if anything, more sense in this world than it did in ours; the extreme, dehumanizing sexism which always points up the escape of the heroine is no weaker in Spencer's reversed formulation, and the root need is species survival, not inherited wealth.
It's no better a system, of course. It fits my loose belief that the willingness to oppress classes of people, no matter how much one loves members of the class, may begin with material need but becomes an end in itself. Now, Spencer isn't at all preachy about this, which she partly doesn't have to be because we knoooow that oppressing men is wrong - Man bites Dog - and partly doesn't have to be because it's also a subtext of lots of romances (pretty much all the ones with spunky heroines who can ride, as opposed to tender ones who can suffer).
I have a mental test for said romances, kind of like the Mo Movie Measure, which in fact I apply to wish-fulfillment literature in general; is the gift (love, superpowers, inheritance) used to amend the injustice? In Regency romances, the Improbably Ethical Endings usually have her dowry legally under her control, or his money and power used to protect orphans and legless veterans, or so forth.
In A Brother's Price, it only helps one person. In fact, his whole life is charmed. So the adventure/love story is restful, but not interesting.
The worldbuilding is great, though. My theory about the cause - all spoilers from here - is that syphilis, which does damage pregnancies, mutated to be almost always lethal to male infants. That would cause the sex imbalance. Even societies that understand transmission don't control STDs, so syphilis would still be endemic in the population, serving as a motive to value chastity. The second zinger Spencer adds makes it just imaginable that most families actually maintain chastity; family successsion from cohort to cohort of sisters, one generation the mothers of the next with the husband they all share. Now, this is a solid fix from the point of view of the gene, which is why bees and maybe lions work this way. It's also an imaginable human society, because each sister has to stay clean or her sisters will catch it through their mutual - and irreplacable - husband. Husbands don't have the power to enforce chastity, any more than wives can in patriarachal societies; but sisters and mothers do. Law and economy have to change to reflect this; basically sisters are legally one person - they might get knighted, for instance, and become the Sirs Lastname, or they might all be executed for the treason one of them commits. Very nice. Finally, the technology is plausible assuming that this variant of syphilis arose in the discovery of the New World and wiped out nearly everyone, leaving a very weakly European society to regenerate over some hundreds of years.
Find in a LibrarySo wrote clew in SF&F.