IMHO, I think that the little kids are dressed alike because they are little, I don't think it has anything to do with they are royal. As much as we know about the Princely family, I bet they let Marius decide what he wanted to wear as long as it was appropriate for the pictures. Being almost a teenager, I wouldn't have wanted to have to dress like my little sister and brother, because that is what they are to him, no matter what their royal status.
I would of also loved to see all the family together but hopefully we will see M-L's family today.
Merry Christmas Everyone!!!!