W P Baildon speculates, Baildon and the Baildons p. 354, that both his wives, Maude and Isabel, were Deincourts, thus accounting for sundry transactions between Deincourts and FitzWilliams from as early as 1313.  But he also concedes some uncertainty about her name.

Baildon is of the opinion that ?Maude was the mother of all William’s children.