The earliest Clan Mother of Dathomir was the banished Jedi KnightAllya, who literally sired the first witches and taught her daughters the ways of the Force through the lens of magicks. A Dathomiri Clan Mother was the heir of Allya's authority over the clan members, known as Sisters.[3]
For a mere Sister, a convenient way for the echelons of power was to kill the current Mother(similar to the Sith's typical method of succession). If the assassination attempt was successful, the murderer was considered worthy of taking her victim's place. Around 31 BBY, the young Ros Lai stabbed QueenZalem with a lightsaber, to which other witches reacted by saying: "The Mother is dead. Long live the Mother."[6]
"You become the Mother of the Hiironi because it is what the irstat wants. What you want is not important. What is best for you is not in your hands to decide. You are part of something much greater. We are Tarasin."
On Cularin, a "Mother" was the eldest woman of a Tarasinirstat. This woman was considered a leader and wise-woman, usually having served for several years as an Irstat-Kes.[7]