ok The sage before dying had 2 children indra and ashura.
Ashura had the sages body and humongous chakra pool ok.
While indra (if that's his name sorry if I got it wrong) Had the sages eyes aka the uchiha right.
But then when we learned of the uzumaki's and senju's being distant relatives that tell's us one thing. They are not direct siblings so they have to be cousins. And where else could these cousins come from? Well that's were the sages brother cames in if he had children they could have been the uzumaki and the hyuuga.
I think this because
1.The senju and uzumaki share similar traits like the extremely high chakra pools and stamina.
2.The hyuuga and uchiha are the only clans to have a dou-jutsu that changes their eyes slightly or greatly. (slightly because the byakugan is barely noticeable except the huge veins and black lines in the eyes.
But since their are distanct relatives they had to have come from the sages brother because if the sage didn't have the uzumaki's and hyuuga's it had to have been the brother if this all makes sense?