Board Thread:Naruto Discussions/@comment-3568954-20140803003603/@comment-26302001-20140803033729

Eternal MS, is only granted by implanting another Mangekyo Sharingan. In the Uchiha Clan, the first to awaken the MS was Madara then Izuna and apparently there have been more after them. Through trial and error the EMS came to fruition, apparently Madara and Izuna only decided to try EMS after the others failed.. they studied the others efforts and realised only a close family member such as siblings for one example, could get the EMS. So truthfully.. unless Obito and Shisui are at least half siblings.. then the EMS thing could never happen for them anyways. It would be cool though if Itachi was brought back and he took Sasuke's old eye out of the mountain graveyard and got EMS himself.