It is interesting that the Treaty of the Maroccidens declares that only Republics can be in the Veinsormoot, yet one of the three founding members is a Monarchy. 
We don't require that you
be a republic.No realm shall be admitted to this agreement which has not adopted a republican style of governance.
We require
a republican style of governance.D'Hara is not formally a republic maybe, but they have regular voting on many positions (including, crucially, the ruler), give the most significant legislative power to the lords, and generally feel like a republic to the politicians in Terran.
