As far as I know, these banners are historically accurate. Don’t forget though, this castle is not Flemish - it is Luxembourgish. The town of Vianden is a real place and it is located in northern Luxembourg.
However, on the point of historical accuracy, we haven’t got everything perfectly right in the past and so I will double check with our historian, just to be on the safe side. Thank you for your feedback.
EDIT (11/03/2023): I have just received feedback from my historical colleague that, yes, the banners are historically accurate and were researched from sources held at Vianden. Even still, thank you very much for your question. I do hope that you enjoy wandering around the castle.