Dinie Netjes, my oldest sister, is married to Siek Sieperda. Because this family is originated in South-West Fryslân, relations have been searched with the eight main families. They exist. The list of ancestors has been investigated by André van der Veer from Beetsterzwaag, his mother-in-law is a sister of Siek. This information is here on the site, thanks to him.
Sieperda, Syperda en Sijperda are all pronounced the same way, these names are also related. The original Frisian way of writing the name is Syperda. Early in the 19th century some called themselves Sieperda.
Bauke (Oene's son) Syperda had 13 children between 1815 and 1845, of whom only 4 reached adulthood and had children themselves. In GenLias and in the RijksArgyf these children are called Sieperda and Syperda. Most of them have the name Syperda, but with marriage or death the name appears to be Sieperda. This difference appears to be dependent of who registered the names at the time.
Typing all of the ancesters is not finished yet, I have more generations, but not enough time. The database now contains 1600 people belonging to the Sieperda family.