@tuatara1 I doubt it being a Japanese and Japanese media's thing tho. It might appear like that because of the limited exposure of Asian media due to language barrier or interest. I mean, the "old virgin woman" stigma/trope can still be seen not only in J-Drama but also in K-Drama and C-Drama, and I dare to say in other Asian media too. Although, I think both Japan and Korea are now being the lead on abolishing that stigma based on the seemingly increasing number of female being single in their 30s in those countries. If any, this trope usually ties to being old-school and this is why I have doubts about your statement.
For the story itself, it is set in Middle Age tho. I have no idea what kind of value did people back then have during this period, especially in the European continent. However, I can see why would the author think that such old school thinking might persist during this period of time.
I do agree that familial hierarchy titles do not correlate with age, they are just a name to define the relationship of two people in the family tree. However, I have seen some people who took those title seriously and would prefer to be called sis/bro or direct name calling instead of being called aunt/uncle. They might be a minority but this kind of people does exist. The problem might be come from how overly used this trope is in manga as a comedic value. This part is quite subjective and that's what I can say about it.
About the jealousy story trope, well, Asian people in Asia like to compare themselves with other in all aspect of life consciously or unconsciously so there's some truth in that trope. Also, looking on how the author using stereotypical archtype in his story, I think having a better female characterization is just asking too much. Alternatively, it is just an overused comedic trope and we're thinking too deep about it.