Each tribe often has their own separate treaty and retains their own individual status as a domestic dependent nation. So, yes, they have the right to each make up their own purity laws, and often do depending upon their situation. Some nations haven't done so well, and are dying out- some of their laws literally allow tribal members to supersede the rights of an individual family and keep their children on the reservation. Others have done too well, and gotten greedy and don't want to spread the wealth, and have kicked other members out for various reasons, often along genealogical lines. The amount of standing the BIA has over these rules depends on the treaty signed and under the circumstances historically surrounding that specific treaty.