Usually, nobles have all their children placed in a government registry upon birth. It's for inheritance and marriage purposes. Also to keep tabs on the nobles in general by the monarchy. So it wouldn't take a magic appraiser to know that there was a second child in the house, he probably only went there because the registry said there were two children who came of age.
I'm curious, did they say her father was dead? Or is there some other reason he is absent. Does she take after her father in looks? What the deal with that? I mean they keep saying Baron's wife, Baron's daughter, but as the older twin, if her father is dead, wouldn't she have the right of inheritance? A widow can only act as regent/guardian until the heir comes of age.