Supposedly, this Katherine (not to be confused with the Abbess of Barking, same name, different parents) was firstly the second wife of Sir Miles Stapleton (married probably c. 1440), by whom she had two daughters, Elizabeth and Joan. (Elizabeth was *supposedly* 30 in 1488 and Joan 26 "and more", per their mother's IPM - but Elizabeth was said to have been married to Sir William Calthorp "before 7 March 1464" (and birth estimates for their children start in the late 1450s), which suggests that "30" was a very bad estimate. She was probably over 40 (but may have looked younger).
The situation is even worse with Joan,, whose *first* husband was Sir Christopher Harcourt and whose children with him must have been clustered between c. 1465-1474. - so the IPM WAG on her alleged age was even farther out of whack. She is *not* likely to have been born later than 1450, which means she must have been well on the high side of 30 and nearing 40 by 1488.
Joan pretty much had to be married to Sir Christopher before her father's death in 1466, because then things get really weird. Katherine, her mother, having become associated with the Harcourts through her daughter's marriage, proceeded to marry Joan's husband's widowed father! (Wouldn't there have to be a Papal dispensation involved? Canon law often did not distinguish between relatives by blood and relatives by marriage.)
Supposedly, there was a son William of this very late union - but considering that Katherine cannot have been born later than 1416 and that she was now on the high side of 50, it seems a bit unlikely. (There was already a William among the children of Sir Richard Harcourt and his first wife, Edith St. Clair, so there may have been a case of mistaken identity here.)
See this discussion for the birth discrepancies for daughters, Elizabeth and Joan.
https://www.geni.com/discussions/178880?msg=1204244
If Miles Stapleton, V was a son of Katherine, he must have died before his sisters otherwise he would have been Katherine's heir.
It seems highly unlikely Mary Montfort was a daughter of Katherine. She is not named as a co-heir with Elizabeth and Joan.
According to Magna Carta Ancestry 2nd Edition Mary Stapleton was a daughter and co-heiress of [?John Stapleton], by Margaret, daughter of John Stapleton Esq., of Stapleton, Shropshire etc.
https://books.google.com.au/books?id=8JcbV309c5UC&pg=PA258
I will disconnect her and add new parents.