I can't belive I never figured that out before now, but there is a major inconsistancy / impossibility in regard to the age difference.
According to the pirates themselves, when Faris was rescued she was barely talking and saying "Farisa" for her name. That would mean she should be about between 2 and 3 years old when she was rescued by the pirates.
However, in a flashback
we see her at castle Tycoon being at scool and studying magic. She should be at least 7 years old in this cutscene, and she doesn't quite talk like a baby !
Also in this same cutscene we see Lenna which is clearly a newborn baby.
In the Final Fantasy wikia
they say Faris' age is 20 and Lena's age is 19, but I don't know where those number comes from. Since Faris lies on her gender she could lie on her age as well.
The only scenario that "makes sense" in regard to the game, is that Faris was originally about 6 years older than Lena, but she magically became a baby again when she fell in the sea, and after that even she became younger by 5 years and had a 1 year old difference with her sister. However that does not quite make sense at all, even in an universe filled with magic.
PS : Also I've heard of people having blonde hair when they're small and it's becoming dark as they become adult, however, Faris and Bartz definitely are special cases. According to the flashbacks, Faris' hair turns from brown to purple, and Bartz' hair from green to brown. However, in an universe where people can have green / pink / purple hair, I guess that makes sense...