Use the following method it will give you notification for only the faction that you are playing not for the AI princesses.
Put this script in the Campaign_script.txt file found in this location:-
....mods\SS6.3\data\world\maps\campaign\imperial_campaign folder
Code:
monitor_event CharacterComesOfAge FactionIsLocal
and AgentType = princess
add_events
event historic princess_notify
date 0
end_add_events
end_monitor
Then put this code in historic_events.txt file found in this location:-
....mods\SS6.3\data\text folder
Code:
{PRINCESS_NOTIFY_BODY}Blah Blah A princess has come etc etc....
{PRINCESS_NOTIFY_TITLE}Hot Chick in Town!
Lastly put some pic for the event otherwise you will get the generic pic of that monk guy that we see in vanilla trailer.
Name of the pic should be princess_local and format should be in .tga format and then put this pic in this location:-
...mods\SS6.3\data\ui\southern_european\eventpics folder
you can use this generic pic if you want:-

Good Luck!