To the best of my knowledge of Hamas, I want to clarify that there is no such thing (in the language of Israel and the pro-Israeli international media): The leader of Hamas in Gaza is Muhammad Sinwar.
The Palestinians in general and Hamas in particular are not a monarchy. There was no king. Yahya Sinwar was not a king and he did not crown his brother as the leader of Hamas in Gaza. Muhammad Sinwar was never part of Hamas’ political leadership.
There is no single decision-maker in Hamas and there never was. Decision-making in Hamas is much more complex than that. In Israel there is a single decision-maker, not in Hamas.