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Mother of the Bride
Have you tried bridal shops already? They do have a good range usually of MOTB outfits and evening wear. Antwerp and Leuven might be better places to try than centre of Brussels (going on my experience when I got married, but that was a long time ago!).
There are a couple of boutiques in the Woluwe shopping centre where you might find something to suit, or try Inno.
What about looking on-line too ? I have been invited to a wedding next summer and am already looking for ideas on sites such as Phase Eight, Fenn Wright and Manson, Hobbs. Also Net-a-porter, although their prices usually bring a tear to the eye, but last time I looked they had some nice Issa desses that were quite reasonable (well for a special occasion that is !). Also Zalando.be have a designer section.
Good luck and happy hunting !