--Previous Message-- : She seems to wear a brooch on the right which : looks like the lower part of Wilhelmine : stomacher
As Gert-Jan pointed out in one of the fotos, the lower part of Wilhelmina's stomacher has nine prongs. So I guess it's the other way round: The aigrette in Irene's hair is the lower part of the stomacher and the central part of the tiara is Irene's brooch.
The question remains: Is the upper part of the stomacher indeed the well known large brooch?