Selene's affinity rises to MAX simply by completing the two quests relating to her, after which she relocates to your home in Cassardis. And Aelinore's affinity can be raised to MAX simply by talking to her repeatedly the first time you see her (outside the castle, after meeting the Duke for the first time). Personally, I found the best way to ensure the "beloved" is the person you want is to make sure that no-one else has MAX affinity, and the easiest way to do this is to lower your affinity with them by punching them repeatedly. Although in the case of Aelinore, there's a way for her to be killed off permanently, as you likely already know. Also, note that you can still change your "beloved" between the Dragon battle and the end of the game; and that if you gave the "beloved" the Arisen's Bond, a cutscene shows this ring falling from the Dragon as it leaves the Greatwall (which is to say, since this ring didn't drop for you and you gave the ring to Quina, Quina wasn't your "beloved").
See
http://dragonsdogma.wikia.com/wiki/Beloved