Willem's will was in Dutch and is of record. It is dated 11th December, 1716 and resites him as of Dutchess Co. It was proved 29 October, 1722 (See NY Calendar of Wills, p. 382). It mentions the wife and the 10 children, naming, in order, the sons: Abraham, Stefanus, Jacob and Isaac, and the daughters Elizabeth, Eghie, Rebecca, Marytje, Helena and Areiantje. The witnesses were Henry VanDer Burgh, Elias Van Bunschooten and Leonard Lewis.
For the history of Nieltje's father, Theunis Cornelisze Swart, consult my manuscript volume, uniform with this, entitled "Some notes as to the early descendants of Theunis Cornelisze Swart," soon to be presented to the Lenox Library, NY City.
Willem Tietsoor was assessed on £13.0.0 1717-18 (lst Supervisor's Book, Dutchess Co., folio 3. Published by Vassar Bros. Institute). Cons. folios 11, 26 etc. of the same book. "The Estate of William Titsoort, Deeceased" was taxed on £6, August 9, 1722 (page 57).
Consult the magnificent "Swarthout Chronicles," pp. 140, 145 which call Willem the earliest NY settler on a Minnisinck farm.