You mean between the FL and her little half brother? He's 10 years old. The mage is 20. And the warrior is 19. Physically the FL is around 14 (she said so in the first few chapters). She may be older since she's a half elf but we don't really know how much older. I'm just gonna assume she ages slowly or some elven shit like that. Since she is only their half sister, they don't have elven blood and are the age they are.
She is at least a year older than the oldest depending on how long it took the emperor to marry and have the oldest child of the empress. The smallness of MC is likely being a half elf and poor living conditions since childhood have left her a bit stunted.
We don't know her true age so it’d just be speculation. We only know that she's physically around 14 yo
Why do they cut out important explanations tho? They should've explained the age difference between the siblings if the fl is supposed to be older than all of them? Like.. it's not as obvious as the author wants it to be idk..