Board Thread:Fanon Canon 2/@comment-2001574-20140921163723/@comment-6003865-20140923041322

Because the Rinnegan was wielded by the Sage of the Six Paths, merging the chakra of his two sons, Indra and Asura, within one's body brings forth the Sage's chakra, which can lead to that person awakening this dōjutsu.

just says you need the sage's chakra, does not say you need the sharingan.