Hello, I believe the parents on this Joan FitzWilliam are incorrect. The Joan that married Brian is the daughter of William FitzWilliam and Isabel Deincourt.
Joan (dau of William FitzWilliam and his wife Isabel Deincourt) married Brian de Thornhill
Source: https://books.google.co.uk/books?id=kjme027UeagC&pg=RA1-PA106#v... Page 105
