Must protect at all cost. She's been though enough.
Imagine the uncle demanding respect for being a mass of garbage. You insulted the wife, you should be glad I didn't choose the knife.
So, fun fact: the idiom "Blood is thicker than water" is a shortened form that distorts and flips the original phrase: "The blood of the covenant is thicker than the water of the womb". That is, relationships we choose willingly are more important than those we are born into. While Western marriage vows are ultimately from medieval England, if we assume that this manga is a period piece/
jidaigeki from the clothes, Edo period marriages still had the wife legally join her husband's household.
Omiai and Shinto marriage ceremonies came later (in the Meiji period). And while love was less important in this period, if the husbando is of the samurai caste, then protecting his wife's honor is also protecting
his honor.
tl;dr: So, whether before God, the daimyo, or the kami, the husbando has made an oath that implies putting Aki in a closer relationship than a dirtbage uncle. Therefore, he has a moral obligation to protecc the smol oni wife.
