Old king's sister was Judith's grandmother, Vanessa was the old king's daughter (Jonas' mother, Duke's dead wife). Current king is Judith's cousin, old king is Judith's... great uncle? I guess? Vanessa would have been a cousin to Judith as well.
Luca's family relation is a lot more complicated than Judith's. He has his great grandmother on his maternal side (old king's sister) then on his paternal side he has his grandmother (Vanessa, who to his maternal great grandmother/Judith's grandmother would've been a niece). There's definitely a little bit of distant incest going on there but IRL incest doesn't show consequences or any signs at all until a few generations pass and I think that's still assuming that there's MORE incest added and that the relations are close, like brother and sister have a kid then that kid gets with their brother or sister or maybe cousin, etc. On the maternal side Luca is born after two normal couples (great grandmother/old king's sister found a normal husband outside the royal family, her child, not sure if it was Judith's father or mother, also found someone outside the royal family), Judith's sister ended up having Luca with Jonas but the royal bloodline is already like twice diluted with her and Jonas can't really be traced beyond Vanessa and whoever his dad might be but that's normal. Same for Judith, any kid she has with Ruediger won't really have issues since Judith's royal blood is twice diluted and Ruediger isn't of royal blood at all. You did NOT ask for the incest explanation but I added it anyways lol.
am I the only one confused about the family tree? huhu I forgot was the prev king sister (the one that ran away) FL's mom or grandma? I need to write this family tree I am getting lost