Checking through an interesting discussion on the Genealogy.com Marsters Family Genealogy Forum I found a thread that was debating the issue of who was the "Middle Family" and why were they called the Middle Family when they say they are descended from the first wife of William Marsters.
If you read any documented genealogy, and researched material as in the "Stories of Palmerston" or "The Masters of Walcote", you will see that Akakaingaro (who was named Sarah by William Marsters) was in fact married to William Marsters according to pre-missionary custom on Penrhyn island.
I am descended from Sarah's family (my lineage is William I, William II, Tearaia, Jane, me). Akakaingaaro / Sarah's family is often referred to as the "first family" because of his marriage to her. William had a liaison with another woman on Penrhyn before he married Sarah, but even she is not referred to as his "first family" even though Arehata had a daughter, Litia (in Penrhyn, and Ritia in Rarotongan).
It is this acknowedged marriage on Penrhyn that identifies Sarah as the head of the "first family". However, when it comes to the allocation of land her descendants are referred to as the "middle family". The other two (de facto) wives of William Marsters were allocated the beach sides of the island.
The sections are marked out by trees and stone edged pathways. In the middle section are some communal usage areas eg the church, the water tank and their "mountain" or "refuge hill". This is a slight mound in the middle of the island where the families frequently ran for safety during the hurricanes of the early days. Even today it provides the island's residents with some protection during high seas.