It's been a while since you posted:
" know my great grandmother had siblings, some who lived on Chincoteague. Do you know if John William and Dorrie were full or half brother and sister? My father thinks Dorrie's mother's maiden name was possibly something like Dunston and that she grew up on the mainland somewhere around Horntown."
so you may have already run across this information.
Miles & Miles, MARRIAGE RECORDS OF ACCOMACK COUNTY, VA, 1854-1895, pg. 361
Wimbrough, John T. 24, Single, Oysterman, born Acc., living Acc., s/o Griffin & Betsy to Roesy [Dunston] Dunton 22, Single, born Acc., living Acc., dau. of John & Kessy on 30 Jan 1875 at Chincoteague Island by Rev. Parker Bowdin, Sr.