This line of questioning can only be addressed with the real world clues stemming from Japanese religious and cultural references. The rinnegan was explained to be a gift from the gods was it not? If true, then we are dealing with a possible conflict between juubi and another near cosmic or divine force. The ripples of rinnegan appear in the juubi indicating its subjugation right? It doesn't mean it is sourcing the rinnegan right?
Why would the gods give the gift of rinnegan to so6p unless they needed some biz taken care of with ol' ten tails?
I think the juubi will need to be explained and developed from a yet to be revealed relationship it has with something else.
The two sons, really hard to say. Im glad it was raised to questioning and that insightful answers already are offered. My mind is way tickled..