She is married to Guy de Dampierre.
They got married
Child(ren):
MATHILDE de Béthune (after 1230-8 Nov 1264). Dame de Béthune, Dendermonde, Richebourg et Warneton. The Genealogia Comitum Flandriæ names "Mathilde filia Roberti Tenremontensis" as wife of "Guido frater eius [=Willelmus]"[300]. The testament of "Mahaut feme au…Guion comte de Flandre et dame de Béthune", dated Mar 1250, is witnessed by "…monseigneur Robert sénéchal de Flandre, monseigneur Hellyn son frère, monseigr Guillaume de Grimberghes"[301]. The Annales Blandinienses record the wife of Comte Guy as "filiam Roberti advocati Bethunensis", and the death of "Mathildis uxor Widonis comitis" in 1262[302]. m (contract 2 Feb 1246) as his first wife, GUY de Flandre, son of GUILLAUME [II] Seigneur de Dampierre & his wife Marguerite II Ctss of Flanders and Hainaut ([1225/26]-imprisoned Compiègne 7 Mar 1305, bur Flines). He succeeded his brother in 1251 as joint Count of Flanders. He succeeded his mother in 1278 as sole Count of Flanders.
Bronnen
[300] Iohannis de Thielrode Genealogia Comitum Flandriæ MGH SS IX, p. 335.
[301] La Gorgue-Rosny (1877), p. 79.
[302] Annales Blandinienses 1250 and 1262, MGH SS V, p. 31.
https://fmg.ac/Projects/MedLands/nfranord.htm#MathildeBethuneMGuyFlandre