Well, isn't it time for people to get over the theory about aging faster, she just got tired for the high mana consume rate. In the novel the ability got explain very clear later, so Aria wont become old any time soon.
About Cain, in the past timeline, he kill Aria to bury his desire for her "dirty" body, which in this timeline due to Aria try to get his help, he think he actually has a chance. So far no harm done, but we can agree that Cain think with his lower body much more, and not the kind of brother any girl want to see crawling near herself