I was doing some research on a completely unrelated subject when I came across this term: "sororal polygyny."

Given my lifestyle, you would think I'd have come across this before.  The Encyclopædia Britannica article on the matter begins:

Sororal polygyny, in which the cowives are sisters, is often the preferred form because sisters are thought to be more mutually supportive and less argumentative than nonsiblings. A typical rule for sororal polygyny is that the eldest girl in a family marries first and that as they come of age her younger sisters join her as cowives....

I like the bit about the "typical rule" favoring the older sister.  That's the way it worked with Kasie, Augustus, and me.  I guess our relationship is not that unusual after all!