Originally Posted by
paulbee
My guess (and that's all it is), is that the SO6P had both Kids while he was the Jinchuuriki of the 10Tails.
Now, the Older brother who inherrited the SO6P's Spiritual and Eye powers, had an affinity for the Juubi's Spiritual power, hence he adsorbed the Sharingan along with the SO6P's spiritual power. We could say that the spiritual power of the Juubi intermingled with the SO6P's own spiritual power and became indistinguishable from it. Thus the Elder brother who inherited the SO6P's Spiritual and eye power, also inherited the Sharingan.
In the case of the Younger son, the SO6P did not absorb the Juubi's Body, so it did not find resonance in the SO6P, hence the Younger son did not inherrit the Juubi's Physical powers.
If it is shown that the Rinnegan came from the Juubi, then I'd buy your theory. Until then you have to explain how the SO6p stopped the Juubi without a Rinnegan. At least with the Rinnegan, he could negate its chakra blasts, and pull out it's soul from its body.