It has been established that Mary Launce's mother was indeed the daughter of Lady Isabel Darcy. Lady Darcy left property to a Mary Sherman living in the US in her will. See Ancestral Roots of Certain American Colonists Who Came to America Before 1700, 8th edition, by Frederick Weir, on Ancestry.com. There's also a book tracing her ancestry back to the Plantagenets, also on Ancestry.com.