It is quite possible that the Uzumaki clan is ancient and the So6P was someone who left the clan along time ago or was someone who was a well known priest but the scriptures of his existence in clan were destroyed.
I am just having one problem about the sage gaining the Rinnegan after he became the Juubi's Jinjuriki, If that was so then how did one of the sage's sons inherit a Rinnegan like eye. Now I have two possible out comes I had thought of:
Tailed Beast inheritance
It is quite possible that the children a jinjuriki has gain some type of inheritance from the Sealed Tailed beast inside them, an example would be Naruto Uzumaki's whiskers. If that is so it would explain the Elder sons eyes.
The Sage's sons are related to him by blood
Well the problem with the theory above is if the elder son gained special eyes why didn't the younger son inherit any features? Evermore why didn't none of them possess any Uzumaki features (red hair, huge chakra pool, etc)? Well it is quite possible they didn't inherit anything because they are not related to him, they may in fact just be his disciples or students, and the eyes the elder brother inherited are the last of the Juubi's chakra that the So6P sealed within his son, this would also explain why it has filtered down over the generations.
On a off note if considering the theory above why the elder brother got jealous of the younger one. Well it is quite simple really the younger brother may not have gotten any power upgrades because he didn't need them and he was instead given his fathers blessing to go and bring piece to the world, which in turn made the older brother jealous.