Also, for Siebelts, I could not find anything, but it sounds similar
to the German word Zwiebel (onion), which is also a surname. There is
a variant of it that is closer to your's: Ziebell. German: metonymic
occupational name for an onion grower or seller, from Middle High
German z(w)ibolle, zwibel (from Late Latin cepulla).
This is just a guess though. I'm no Professor Udolph.