What if Obito is the elder son?
Before my theory is dismissed, take a second to consider that Sasuke hasn;t been named the elder son in the manga yet, so everything is still up in the air.
Personality wise, Obito has more in common with the Younger Son. He wasn't naturally talented, had a personality like Naruto's, and his overall intention was to end painful realities for others not just himself.
I think he has more in common with the Sage than Indra personally. He gathered the bijuu (against their will admittedly), became the Juubi Jin, is the sole reason for Naruto's childhood situation and why Naruto chose his path, and now the Naruto has taught him that cooperation is true power and Obito is now entrusting his Will and the Bijuu to Naruto.
Sasuke fits the bill of Indra closely
and also has the "brotherhood" connection to Naruto the Younger Son.