
The younger brothed objecting was a mistranslation. Original text was his younger SISTER would object. He only has his father and the one sister :) It's been stated already that the Hertan father and his daughter died in the fire, while the eldest was away, the youngest is the only one alive and in town.
Nice theories, tho, the best part in these kind of stories is trying to weave all the info

The eldest is away, yes. The Hartan family (nobles of the town): Parents (dead in the fire), eldest brother (a knight, away, came for the burial but left because of a letter, will be back in a few days but doesn't want to inherit, wants to rise as a knight instead), eldest daughter (would have inherited, but died in the fire), and youngest brother (the one they talked to, wasn't in the castle when it burned so he survived, only one left to inherit).
Devran's family: Father and younger sister, who they say died in their house in the fire (but Yuder just looked and there are no traces of anyone dying there). The younger sister was the reason Devran worried about not being able to move to the capital (they just found a woman's brooch, expensive, with a knight's portrait inside the burned house).
Why do I feel like Devran is somehow connected to the house Hertan? Either he is a bastard child, or something else. Because at the end of the chapter a big plot point is that Devran had a father, sister and younger brother. He wanted to relocate his family to the capital but his YOUNGER BROTHER would object. The shady guy from house Hertan that survived is also the youngest son. Also the shady guy said he had an older brother and a sister. Either Devran is the oldest and refered to the oldest Hertan sin as his younger brother, or he was talking about the shady guy we saw. Either way he is somehow connected to their family.