I did find William and family in the 1910 Census, and Elah in 1920, where she answered that she was widowed.So William did die between 1910 and 1920.The 1910 Census said that William was 50, which would place his birth year at about 1860.It also stated that he was born in Missouri, and that his parents were born in Kentucky.
I looked in earlier censuses, and I was disappointed not to have found William in the 1900 Census.That census gives the birth month and year of the individual, and earlier censuses tend to be more accurate.
I did find a close match to your ancestor in some even earlier censuses.There was a William N. VanSickle that was born in Missouri in about 1850.He was the son of John and Nancy VanSickle.They lived in Macon County from about 1850, along with other VanSickles that appeared to be John's brothers (William K. and Henry).John was from Indiana, but Nancy was from Kentucky.I honestly believe that this is your ancestor's family, but due to the 10 year difference in age between this William N. VanSickle and your ancestor, I cannot be certain.
I've been doing genealogy for a long time now, and censuses are often wrong.For whatever reason, William may had been enumerated incorrectly in the 1910 Census.I think getting William's obituary and/or death certificate is the only way to find the names of his parents.If your ancestor was from this line of VanSickles, I feel confident that the informant on the death certificate (probably Elah) knew William's parents.They were in Macon County for a long time, and John was still alive in 1900.
Good luck, and please let me know if you find William's parents.Even though I doubt that I am related to your line, I am interested in the Van Sickle name in general, especially those lines that have not been thoroughly researched.