Responses

not a niece, i think the MC and vienna have the same father. My guess is that after MC's mother died, MC's father remarried another woman and had vienna. So Vienna is legitimate but has no right to the Orsa name (since it's MC's mother who was an Orsa and the father married into the family). But it's just a guess, she could also be an illegitimate daughter. I just tought she would not be so easily aknowleged by society if she was illegitimate
Maybe Vienna is the marquess' illegitimate sister?