Quill's comment regarding Quain's "family plan"...I honestly think that this was Quain's twisted way of trying to make up for her mistakes with Mel. At this point, Quain likely feels that there's no chance of recovering any loyalty from Mel or any way to salvage their relationship. Syphile's dying words seem to have hit a nerve, and one of the best parts of the following chapters was where it showed Quain make an honest effort to do better with Ariel. Whether it be defending her from Suu'be, or taking a wolf ride, and later on when they're shown sparring after the timeskip, Quain is shown to be harsh and ruthless but not half as bad as how she treated Mel and Syphile.
I have a suspicion that Quain may have realized Ariel's attraction to Faen and made the decision (especially after getting cut by Sarv'swati's poisoned blade) to repeating history. By giving her Mel's children, she would not only be following the tradition of having one of the daughters raising the younger siblings, but it would also give Ariel and Faen blood-related children of their own to raise without forcing Ariel to have to mate with a male if she didn't want to. This would avoid a lot of the family issues that she kind of created in the past, and allows a compromise to make her heir happy and continue the family line. The only person who really suffers in this plan is Mel, unfortunately, though I feel that Mel would happily endorse Ariel and Faen raising the kids if it kept them away from Quain and kept Ariel from going through what she did in order to satisfy Quain's desires for a family line. At least, she probably would if she knew that they were all alive, that is.