Not sure if this is what you are looking for but, per Dorman,
Thomas Willoughby b. abt. 1600 wife Alice
Children Thomas (b. 1632), Elizabeth (b. abt. 1635)
The second Thomas was the father of another Thomas and Elizabeth
The third Thomas married Margaret, their children were
Thomas, Margaret, Sarah, Mary, and Elizabeth
Mary married Moses Robertson in 1732, they had one child a son John.
The only other Mary Willoughby listed in this family was the daughter of John Willoughby, the son of the fourth Thomas. Although he names spouses for all of Thomas' other children, none is listed for this Mary.