Siblings brandish great power in the family- as poignantly described in Allen Shawn’s new memoir Twin, reviewed in the New York Times.
Family systems theory treats the family as a set of separate relationships that are interconnected, then uses this to try to understand how the family functions. Siblings are a subsystem among other subsystems like parent-child and marital subsystems
These other system over time, affect the sibling subsystem and the sibling subsystem affects them.
The uniqueness of the sibling subsystem is that other subsystems may dissolve because of death (grandparents and parents) but the sibling subsystem lives on, the longest surviving subsystem. It only breaks down when the family of origin dissolves.
The sibling subsystem also moves out into a highly autonomous system in adulthood, away from the interdependent family of origin. . These sibling subsystems can be dyads, triads or larger groups.
They wield large power in the family by aligning and exercising their clout based on alliances that meet their needs. So as the siblings may be able to get together and weasel Mom into lining up with them and getting Dad to allow them to go to the movies when they are young, they have similar power in adulthood. A dyad of siblings may align to get Dad to leave another sibling out of his will or convince Mom to go into assisted living against the will of another weaker sibling.