the elder son wasn't born with the yang chakra like the younger son so its just an ems
he was born with an ems because his blood isn't thinned, he doesn't have to get a siblings eyes for this
when sasuke first used his ms his chakra resembled genjutsu which is yin element he then got itachi's eyes and more yin chakra to equal the yin chakra of the elder son and his eyes,the elder son's eyes look like that because it was the first distortion of the rinnegan
if you have an ms and get yang chakra it won't evolve to rinnegan cause it's half of the yin chakra of the elder son,like obito's
but ms(which comes with yin chakra)+ms(which comes with more yin chakra)=equals the yin chakra and eyes of the elder son + getting what the elder son lacks(yang chakra) =rinnegan