Answer
Ptolemy III Euergetes, the second Greek king of the Ptolemaic dynasty, had no siblings. His father, Ptolemy II Philadelphus, had only one child, his son Ptolemy III. This is because Ptolemy II married his sister, Arsinoe II, who was his full sibling. This was a common practice among the Ptolemies, as they believed that it would keep the bloodline pure.