Complex sentence: Peter II was declared as King of Cyprus after his father's murder in January 1369; however, at fifteen, he was still a minor, so his uncle John of Lusignan, Prince of Antioch ruled the Kingdom as regent until Peter came of age.
Split into simpler sentences: Peter II was declared as King of Cyprus after his father's murder in January 1369. However, because he was not adult (15 years old), his uncle John of Lusignan, Prince of Antioch ruled the Kingdom as regent until Peter came of age.