I'm not answering your question about your grandfather's siblings, but here's the list of WW I draft registrations that may help you.
Martin Kuretich, (b) 15 Sep. 1873, living Delta, CO,
Anton, (b) 17 Jan 1876, living Gunnison,
John, (b) 23 Sept 1876, in Delta,
John (b) 24 Oct 1876, in Gunnison,
Martin (b) 14 Dec 1877, Gunnison,
Frank, (b) 4 Apr 1884, Gunnison,
Frank, (b) 1 Sep 1893, Gunnison
Mike Frank, (b) 6 Aug 1898, Delta.