Aria ends up using him for a bit but he eventually gets... impatient? and lashes out at her. I can't remember how that incident is solved, but I do know that Cain ends up dying by Mielle's hands. Mielle intends to poison Annie way later on in the story, long after Isis' head has already rolled, but Aria sets her up so that Cain ends up drinking the poison. Aria by this point had already """"saved"""" Mielle from being executed once, after having pushed their father down the stairs and basically joining that bitch Isis in attempting a Coup. (Coup d'etat). Mind you, Aria only saved her to let her suffer a little longer, and by that point in  the story everyone was on her side.