First off Targaryens banging out relatives is nothing new, Danny's parents and grandparents all did it and in the first season her brother was doing it to her as well before selling her to Drogo. Second the series has already surpassed the books so you lucked out there, no spoilers forthcoming.Considering the hints the show has dropped about who Jon Snow's father is likely to be, Daenerys and Jon banging would be a bit odd.
No spoilers from the books please. Clearly the show has hinted strongly, but has not provided confirmation yet.