I think her brother is Roelof Claasz (birth also unknown), because Divertje is a witness for Roelof's children. Roelof was married to Divertje Sijmens in 1753 (children: Joanna in 1755, Simon in 1757, Maria in 1759, Geertje in 1761 and Nicolaus in 1763).
Any good guesses for their parents (presumably their father is called Claas or Nicolaus)?
When inquiring into Dutch families before 1812 it is imperative that you mention their religion, as baptism and marriage records were recorded separately for each denomination. I see now from the links they were Roman Catholics. From the fact that the first two daughters were called Cornelia and Aaltje it maybe that the two grandmothers were called Cornelia and Aaltje. So, you may want to search for a couple Claas and Cornelia or Claas and Aaltje, who had at least two children: Divertje and Roelof.
The fact that Roelof named his second son and fifth child Nicolaus and had no daughters Cornelia or Aaltje makes him a less likely candidate to be a brother of Divertje. And the patronimic Claas is very common in West-Friesland. But it doesn't rule him out.
As Divertje married in 1756 a fair guess is that she was baptised in ±1731 ±5 years. So that is the year range in which you should search.
I get the impression that the BMD records of Grootebroek have not been indexed that far back yet, so you will have to go through the RC baptism records page by page.
Bij het doorbladeren van het RK doopboek van Grootebroek kom ik de naam Divera/Diver regelmatig tegen maar niet in combinatie met een vader die Claes/Klaas heet (of ik heb het gemist).
Wél een doop gevonden met een "Diever Claes" in buurgemeente Wervershoof.
Daar wordt gedoopt op 18 februari 1736: Diever, dochter van Claes Wit en Marij Ariens. Doopgetuige is Griet Ariens
Thank you Michael, M Jacobs and Liesbeth. I couldn't find any positive leads in the baptismal records of Grootebroek from 1729-1736. I will look more closely at the records you have provided. Thank again!