Hodierne de Gometz-la-Ferté - Father unknown

I can see your point, as the Foundation for Medieval Genealogy concurs that they were siblings, parents unknown.

http://fmg.ac/Projects/MedLands/parcorroc.htm#HodierneGometzMGuyMon...

ALSO, in this French language academic paper by Raphael Bijard, "The first conflict of the Capetian era - First Capetian conflict" (2020), the author included a genealogical chart to be seen about 3/4 of the way down from the top -- with Guillaume prominently circled and Hodierne shown next to him quite plainly represented as his sister.

https://www.academia.edu/42907629/Le_premier_conflit_de_l_%C3%A8re_...

"Given the chronology, Hodierne is of the same generation as Guillaume – which makes her his sister if they are close relatives. In Continuation d'Aimoin, she is called Hodierne, lady of Gometz and La Ferté. But as for the lady of Nogent, this late name has no probative legal value, it translates here its origin. Nevertheless, as everything suggests that the second Guillaume de Gometz had no heir, it was through Hodierne that this honor was passed on to his son Guy le Rouge."

https://www.geni.com/documents/view?doc_id=6000000183712332823

https://www.geni.com/documents/view?doc_id=6000000183712221821