1.sage of six paths = yin (eyes) +yang (body)
2.yin = rinnegan =sage's eyes = elder son = spiritual energy = Uchiha (rinnegan = yin and not yin + yang)
3.The elder was born with the sage's eyes (the rinnegan):Manga fact
4. when madara got hashirama DNA he got closer to the sage of six path DNA and that is why his eyes look like the eyes of the sage of six paths.
it make sense that the closer your dna get to the juuubi the more your eyes look like his.