Ok to clear up a few things: Eren's memories weren't the ones altered, it was Historia's, Mikasa's family was slaughtered because a few bad guys thought that girls from the oriental clan would make good money as sex slaves, and the two bloodlines unaffected by the royal titan power that turned against the Reiss family were the Ackermanns and the Oriental clan, the chapel Ymir was at was likely different since the Reiss chapel was a private location