beta
Unbothered, as she should be. Why would her position change from this? She wasn't adopted to replace a "real" daughter, the duke didn't even know he had one. Her mother married the duke legitimately and she and her brother were adopted as a result. The new daughter's appearance doesn't dissolve the duke's second marriage, so it also has no impact on our girl's position.
In conclusion, those noble girls are dumb as bricks. If you're gonna try to verbally humiliate someone, make sure it at least makes sense.