According to the german wikipedia page Sieker had 1.910 habitants in 1885 & 7.320 in 1910. I would presume it was a village . Not a town.
The state of prussia established a documentation of birth , marriage and death records in 1875. Prior to this certificates had been released by the churches where the event happened.
Probably it´s best to contact the church archive in Bielefeld.
http://www.archive.nrw.de/kirchenarchive/evang_kirchenarchiv...

They store, conserve and restore churchbooks of this area between 1600 - 1981.