Calling it now: Incest is Wincest. Dead queen related to her son's fling.
I'm thinking-- one of the earlier chapters mentioned that Judith doesn't quite look like her other family members or something? Maybe Jonas and the Mom aren't related, but Judith is secretly only a half sister to Luca's mother and related to the royal line?
The story went out of it's way to have both Judith and Rudiger unmarried in this timeline and the first, so I'm thinking the author might have put anti-incest provisions in place.