How okay with inter-fucking the Targaryens are is what Cersei used to use to tell herself it was fine to fuck her brother. Then she just was like "I like fucking my brother that's good enough for me." And Danaerys is Jon's Aunt, not cousin. Not that it would matter between the sheets. I wonder if there'll be any sort of proof of Jon's parentage, like documentation that would make Danaerys put her claim aside. Rhaegar was the heir, and Jon is his heir, so Danaerys shouldn't even be in consideration outside of her waging a war of conquest.