Prince Camille is the first prince (elder brother of Fio), he is the child of the kings first wife (with the king). Prince Fio is the second prince and the son of the second wife of the king, she was a foreigner. Luso is a noble whose family has close ties with the royal family, but he is not a royal.
The King is father of Camille and Fio, who are the princes, but is not father of Luso.
Also Luso's mom feed Camille (the 1st prince and Fio's brother) her milk while she was breastfeeding Luso. Likely because Camille's mom wasn't able to produce enough milk to feed him properly. That's the reason Luso is called the first prince's milk brother. They drank milk from the same woman when they were babies
Luso is just the first prince's MILK BROTHER
i think you kind of misunderstood the way fio explained luso's relationship to him because of the way he phrased it ("Luso is my brother, the first prince, milk brother"), basically to break it down, the "my brother" part is referring to Camille the first prince, and the "milk brother" part pertains to Luso (milk brother essentially means that someone has shared the same nanny, subsequently having the same person to nurse them)
it's kinda confusing because of the phrasing, but once you thoroughly read it, you will get fio's point
Can someone explain to me like I'm a 10yo ipad kid who watches brain rot contents since the day they spawn how the hell Fio is the second prince? He has an older brother from the same mother, and yet he's not the first prince, that jerk Luso is the first prince! Like, is Fio's older brother's father is not the king? Was the king marry a widow with a son???