Luso and Fio are not brothers, I think.
Fio and Camille are brothers.
Camille and Luso are milk brothers, meaning they were breastfed by the same one but have different parents. With Camille being a prince, he probably had a wet nurse and the wet nurse was probably Luso's mother. However, there was no mention about Luso's parents and he was in Fio's flashbacks so maybe the queen breastfed them both, which is unlikely but probably not impossible.
Kinda like the Emperor and Consort Ah-Dou being milk siblings in the Apothecary Diaries with Consort Ah-Adou's mother being the young Emperor's wet nurse.
I'm still kinda confused since I remember Fio calling Luso a prince (?) Hope the author drops the family tree (〜￣△￣)〜
Did you mean this part when Fio clarifies who Luso is?
https://www.mangago.zone/read-manga/the_secluded_sorcerer_wants_a_peaceful_life/uu/to_chapter-6/pg-1/
It is a bit confusing. It's probably translated that way because it's closer to the tone in the raw. Not sure though, unless I read the raw. Shagar's reply in the next speech bubble does point out Luso's connection to Fio's brother. That's how I understood it.
Tbh, it's freaking good!! I was iffy bout their dynamic at first with Luso being involved with Shagar first then his brother, Fio, entering the scene for the plot and then it all unravels like WHUTTT.
Anyway I was smiling throughout reading this because the banters were just breaking typical cliche conversations and one thing I can say is, they match each other's freak XD I feel bad for Luso, but man fumbled hard fr (￣∇￣")
The world building is so good as well, I would love to see more from the author, in a similar verse ヾ(❀╹◡╹)ﾉ~
9.8/10 ♡(> ਊ <)♡