User blog comment:B214/Question and Answers 3/@comment-117.222.4.149-20140804082607/@comment-1359322-20140804084253

First of all it's Dragon Aura not ki. Second, no, the other girls can't, Rias and Akeno both take turns to scatter the Dragon power in Issei's body, so only both of them are affected by it. As to why the other girls don't help in scattering the Dragon power/aura, it's probably because they don't know how to. Rias and Akeno had to research on how to scatter the Dragon power in Issei, so it's probably a complicated process.