the other princess that i know was geunevere su britannia and carin le btitannia.. they have been featured in code geass r2 in the scene when he became an emperor. here are the lists of the imperial family of britannia Britannian Imperial Family
* 91st Emperor Alec la Britannia * 98th Emperor Charles zi Britannia o First Prince Odysseus eu Britannia o First Princess Guinevere su Britannia o सेकंड Prince Schneizel el Britannia o सेकंड Princess Cornelia li Britannia o Third Princess Euphemia li Britannia o Fifth Princess Carline le Britannia * Imperial Consort Gabrielle la Britannia o Third Prince Clovis la Britannia * Imperial Consort Marianne vi Britannia o 99th Emperor/Eleventh Prince Lelouch vi Britannia o 100th Empress Nunnally vi Britannia the picture below was the first princess
I do not believe there are any other prince's although in the code geass spin off मांगा Rolo was lelouch's younger twin brother so he could've been a prince. but other than that i'm not sure.
