It isn't the haltingly-delivered narration, clichéd story or competent-but-unmemorable buttfucking sequences which garner MMW three and a half A's, it's simply because the vid delivers on its incestuous promise.
Sort of. After all the requisite scenes are neatly tucked away, Bone delivers the mother/daughter boyfriend-sharing threesome up for the climax. Although the women don't literally have sex with each other, the daughter does spread her mom's ass cheeks open for penetration, and somewhat later, after the guy blows an internal load up Missy Junior's buns, he scoops out the spoo and feeds it to Madame Senior. We believe that qualifies as a form of forbidden love.