She popped as a caster who just happened to be related to the Overlord, but yes, that typically means they're an heir, or royal and can be a proper non popped heir, Lord of Monies. From what we've seen, Casters have a very small chance of being related to the ruler(note the eyes). It strikes me that most have white eyes, and are unrelated.
On that note, if a royal side pops a caster; and that caster is also Royal(read: related to the monarch), if they are promoted to heir, are they heir designate, or heir? Can a caster become a king/queen, instead of an overlord?
Anyway, from what I see, any command unit(except a chieftain, it seems) seemingly has a chance to pop as a caster. But the reason we have yet to see any heir casters, is due to the fact that getting any casters beyond the first few is extremely difficult, and heirs take forever to pop. It's not worth it to produce a bunch of heirs just trying to get a caster one. Usually.
All systems nominal.