You didn't say where you looked for them so I'll provide that info.
Birth and death records from 1889-1905 Sevier County, www.sevierutah.net/index/aspx?nid=136
Birth and death records from 1905-1910 are held at the Utah Archives & Records Service, http://archives.utah.gov/digital/81443.htm
If you're already checked these sources, they could have been destroyed by fire or flood. Successful Searching!