As a younger son, Rupert was originally destined for a career in the clergy. Gerlach I abdicated in 1344, in favour of his sons, except he kept Sonnenberg Castle, where he lived. This castle was to be inherited by Kraft, Rupert's eldest brother. However, Kraft fell in battle while fighting on the French side in the Battle of Poitiers in 1356. It was then decided that Rupert would inherit Sonnenberg.