here's my best guesses:
1) Citro could be either Villager or Tachibana's brother, but he could also be Tachibana turned into a demon somehow!
2)That would be interesting. I would love to find out Villager has true demon blood.
3) I feel like Citro is older
4) Love this question, honestly dunno but this question makes me laugh!
My tin foil hat conspiracy theory is that Tachibana is a isekai individual. And with isekai shenanigans making Tachibana’s body built different, Citro was formed via mitosis so age is more of a conceptual thing. Villager obtained a little bit of the built different dna which upped his acceptance to demon king blood. My gut manifested this theory cuz it’d be a little funny. At least to me
The image popping up in my mind when you said "TIN FOIL HAT conspiracy theory"
1) Is Citro really Villager's blood brother?
To be honest, they really do look similar, Citro, Villager and Tachibana.
2) If yes, do they share the same mother? I mean, tbh Villager's body accepted Mao's blood too easily, it must mean something, right?
3) What is Citro's age? Is he younger or older than Villager? Was he born before or after Tachibana's gone missing?
4) Why the heck are the kids named Villager and Citro, but Tachibana is Tachibana???