"The issue of Hawise, wife of Reynold de Mohun, is an ancient contention." "As far back as the year 1350, John Osberne, the untrustworthy chronicler of the Mohun family, described her as a sister of William Mandeville, Earl of Essex. Dugdale, perceiving perhaps that this nobleman was contemporary with Sir Reynold's father, makes her a sister of Humphrey de Bohun, Earl of Essex, although he professes to get his information from the very book which calls Humphrey her cousin. Others have chosen to describe her a daughter of John Fitz Geoffrey."
For an interesting discussion, see https://groups.google.com/forum/?fromgroups=#!topic/soc.genealogy.m...