User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5774152-20121125033314/@comment-3146930-20121125045302

As long as she has a boy and girl of the seven, I don't think it matters which one it is. Gaea may simply want to use Percy and Annabeth as they were the ones that defeated Kronos. It could be that they pose the worse threat to her plans, as Percy and Annabeth have shown to be a good team and Percy would do anything to protect her. It could just be out of personal preference.