
In many places yes. They have to be at least 3rd cousins and beyond for it to be legal. Although some country's allow it. I think India might be one, but don't quote me on that. I do know it wasn't that rare in ancient china though. Even if it's legal, I'm pretty sure many modern people would still consider it incest. Looking at the situation of this book through, from what I understand from spoilers, it'd be borderline on legal standards, I'd have to physically write down a family tree since It's a bit too complicated to figure out in my head at the moment, but I can figure it out if you really want me to. Also I imagine that some people would call out the incest card regardless of if I find out whether it's legal or not...

Not by blood if you're referring to Ruediger and Judith. Ruediger is the son of Sophia and the duke. If the theories are right then Judith would be the daughter of Princess Vanessa and her father (that's probably not the duke because if it was she'd be with them.) So there would be no blood connection and they weren't raised in a way of them being relatives.
Oh but if you're speaking in general if cousins gets together then I guess it's up to interpretation. I think it's incest and weird if they have any blood connection that not like super distant.
is it even incest if it’s between cousins...? genuinely asking