Queen Paola is known to be an Armani-fan.
Princess Mathilde practicly only wears fashion from Belgian couturiers, such as Eduard Vermeulen from the house of Natan, Gerald Watelet, Pierre Gauthier, Olivier Strelli, Dries Van Noten...
Princess Claire usually wears Natan
And princess Astrid wears all sorts of clothes (I mean also not designer), but also quite often Natan.
Here are some links: