Sub-mod suggestion here.

So I had a Lithuanian Prince with four daughters and a son- the last daughter had the unfortunate luck to turn 14 on the same turn that her father died unexpectedly.

The son is now the new Heir (The current Grand Duke, his grandfather, is still alive) but since the father died, the daughter has now become a non-princess.

Is it possible to give all proto-princesses who have not yet reached their majority an attribute that will cause them to age into princesses regardless of other circumstances (i.e. premature death of father?)

If she's the sister of the crown prince and the granddaughter of the Grand Duke, little miss Not-A-Princess should still be a princess. She's directly in the line of succession.