There's a rumour that he married a girl called Isabel.
But he died without surviving legitimate issue and his estate passed to his cousin (not half-sister) Aubrey de Lisours, then the widow of four husbands, but generally referred to by her maiden name.
Sources
Royal Ancestry by Douglas Richardson Vol. V page 173
Harleian Society. The Visitation of Cheshire in the Year 1580, The Publicatons of The Harleian Society (London: The Society, 1882) Vol. 18, Page 5: "The Barons of Dalton, Constables of Chester. [Harl. 1424, fo. 3b. Harl. 1505, fo. 2b.]" "Rob't de Lacy dedit patrimonium sua sororis suae filijs et in monasterium ses abdidit." (English: ??? he gave his children to his sister and hid himself in the monastery. ???)