sjertsje seems dutch to me too (because of the -tje ending) but because you said something about Kabalarians i was confused
i know that site by the way (
www.kabalarians.com) and it can be very alike (and seeming true) but it's crap at the end to me (found it didn't apply for my name at all, although it did for my mother who changed her firstname and according to the site for the benefit)
it is female name (because of the ending) and i know for sure that there is a male Dutch name called Swiert or something alike because of the surname Swierts and Swerts that are Dutch (meaning son of Swiert)
i look it up for you (to see for more)