Nisan rose out of Nimrod's ashes a little more than five hundred years ago, during Episode IV. Its founding was centered around the founding of the church, and religion has always been an important part of its culture. They are closely allied with Aveh, and they don't appear to have any enemies, although they are certainly still considered a threat by both Kislev and Solaris.
Leadership: Margie Fatima
In the past Nisan has been a hotbed of rebel activity, and they are the only known country to reject Solaris's rule outright and fight back. At present they reject Shakhan and Gebler. The open and forgiving nature of their doctrine and the peaceful attitude of its people are well-known and appreciated.
Social Structure and Leadership
The Holy Mother of the Nisan church holds considerable influence over the governing of Nisan, but the day-to-day political decisions are made by the city council, or greater equivilent. If I had to guess at what kind of government they have I'd say something pseudo-democratic, or maybe socialist, but there isn't any evidence one way or the other.
The clergy is also a heavy presence. The cathedral occupies a centeral position in the city and seems to be a popular pilgrimage site for more than just the Nisan faithful. We have no sense of their hierarchy aside from the distinction between the Holy Mother, head sister, and the normal sisters and acolytes. Nisan doesn't seem like the type of organization to put much stock in elaborate rankings or unnecessary administration.
Religion and Tradition
Nisan is vastly different from the Ethos, in that it teaches about communicating with God from within, instead of without. The founding Mother, Sophia, expressed this as a faith or love that grows and blossoms in your heart, and therefore everyone can hear the voice of God if they let themselves listen. The two one-winged angels suspended above the altar in the cathedral express the human need to work together in order to be complete. The religion is all about love and acting from the heart.
Since Sophia's time there has been a long string of Holy Mothers, and the sect has expanded its efforts to Aveh and Kislev, where they run relief - orphanages, and probably shelters and the like. Their presence also allows them to spy on Shakhan and pass information to Bart and his crew.
After Sophia died, Rene Fatima's daughter became the second Holy Mother of Nisan - the position is hereditary, so the Mother has always been a member of the Fatima dynasty. Somewhere down the line, perhaps after the beginning of the war with Kislev, the tradition of marrying the Holy Mother to the King of Aveh came into practice, and it lasts to this day (though Bart and Margie don't seem thrilled with the idea). I assume the distance between the two lines was great enough that it didn't seem all that incestuous, but the gene pool can only get smaller the longer this goes on. ^.^
Although it is tradition, I think Nisan and Aveh are flexible enough to get rid of it, if necessary. They're remarkably easy-going for an orthodox religion.