Agree Nancy E. Thompson was probably Carrie's mom. Think a simple solution might entail a phone call to Grayson County Clerk? http://www.sos.state.tx.us/elections/voter/cclerks.shtml#M
County Clerks have marriages indexed grooms-to-brides, and vice versa, usually in two volumes (early years and more recent). Request a "quickie" lookup in the index for F. M. Ivy. If my reasoning is on target, he wed Margaret G. Thompson, 24 May 1878, in Grayson? Familysearch has F. W. Ivey but that would not be the first time an "M" was mistaken for a "W". If accurate, that'll also probably estimate the demise of Nancy?