Isabel Mylbery is quite obscure. The earliest evidence we have is from about 1510. Garter King-at-Arms recorded that she was ‘educata ut fert[ur] pre Regem E[dwardum] iiij’ which means, roughly, that she was brought up by Edward IV. She also bore lions and white roses in her coat of arms. None of this is remotely conclusive (only DNA evidence would give us certainty) but taken together it is suggestive. It might also be added that ‘Isabel’ was a Yorkist name of long-standing, borne by the King’s great-great-grandmother, Isabella of Castile, and by his great-aunt, Isabel, Lady Essex.
Isabel Mylbery married John Touchet. He was the second son of Sir John Touchet, Lord Audley, who succeeded his father after the latter died at Blore Heath. Despite his father’s Lancastrian tendencies, Lord Audley, became a firm Yorkist, to the point that he was one of those singled out by Warwick for condemnation. In short, he was a member of Edward’s inner clique. He later served Richard III, being appointed Lord High Treasurer in 1484. (One of the great offices of State.) That such a man should marry a younger son to one of Edward’s illegitimate girls seems wholly feasible. It’s just the sort of marriage such a daughter might aspire to make.
John followed his brother, James, Lord Audley, in rebellion against Henry VII. Unlike his brother, he was fortunate enough to be pardoned. What happened to John and Isabel after that, or if they had children, seems not to be known.
I would like to record my thanks to Brian Audley of the excellent Audley Family History site for drawing Isabel to my attention.