I believe her father Count Patrick bought it himself somewhere in the 70-ties. It is not property with longer connections to the family. His elder brothers did inherit family properties.
Count Henri inherited
Castle 't Couthof in Poperinge. It is the castle where the three brothers were raised. Recently the day-to-day managment has been taken over by Count Raoul's son Bernhard. Mathilde's grandmother Jkvr. Suzanne van Outryve d'Ydewalle had inherited the castle from her aunt and adoptive mother Bss Raoul Mazeman de Couthove et de Tonlieu, née Jkvr. Mathilde van Outryve d'Ydewalle
Count Raoul inherited
Castle Raepenburg in Ruddervoorde from his mother Jkvr. Suzanne van Outryve d'Ydewalle. The castle was built by Suzanne's anscestor the Belgian politician Bernhard van Severen. It is now inhabited by Count Raoul's son Count Bernhard,his wife the historian Jkvr. Marie-Pierre Verhaegen and their children.
An earlier family seat was
Chateau d'Acoz in the Biesme valley, but that was sold in 1860 (acquired 100 years earlier). A few years later the family acquired the right to add 'd'Acoz' to their last name.