i think he was possibly father of the uzumaki and hyuuga clans whereas his brother hagoromo (SO6P) was father to uchiha and senju. making the uzumaki and hyuuga distant relatives to the uchiha and senju.
Since this is a Repetitive Thread, I am going to copy paste. Hope you like it, Dattebayo!
Plot Twist: Rikudou's brother comes back as the Final Villain, destroys Madara, puts everyone under Infinite Tsukuyomi, Absorbs their Chakra, The Fruit ripens, Rikudou's brother eats it and gains Kaguya's Power. Naruto and Sasuke reunite and stop him.