She knows who the vampire is. From Ebenezar McCoy's furious writings in journals about his daughter's company, to Lord Raith's absentminded jots about Margaret Dresden, to the plaque bearing her name in the same way several others were that she knew meant they were under portraits, it wasn't hard to figure out.
From the notes Harry writes to Thomas about groceries and such, she knows they live together. It could be other Harry and Thomases, but she thinks she knows the grinning tone of his voice well enough.
She's happy for them.
It must be nice to have a brother.