It may well be that Agnès de Montpellier is the daughter of
Guilhem VIII de Montpellier
Guillaume VIII, senyor de Montpéllier
and his second wife Agnès de Castille
I know it's not documentation, but wiki records these relationships:
https://fr.wikipedia.org/wiki/Agn%C3%A8s_de_Montpellier
https://fr.wikipedia.org/wiki/Guilhem_VIII_de_Montpellier
Matthew Horton