As you asked this on another message board and received help I will post here so others can know you have results.
Sylvia nee Reeves/Reevis married George Aikman, died before the 1900 census. It seems she was married after 1880 census to George so she will not be found on a Federal Census. If born before 1880 she might be found in that one but need to at least find county in AK born as she maybe listed by Initial or a middle or nick name.
As you were given by another husband George is in Choctaw Nation, Indian Territory 1900 census as a widower and listed as "bro-in-law" in Kuhen(?) household. Her youngest child was born 1897. Her sons William and Gilbert Aikman both list Albion OK as birthplaces on WWI Draft Registration Cards. When they were born it would have been Albion, Wade County Indian Territory before Albion was changed to Pushmataha County OK 1907. So look for where the IT Wade County Court records are stored. Her death may have been registered with the county. Also research old cemeteries.
Gilbert died in CA and is in CA Death Index with mother's maiden name Reeves. It has his SSN and he is in the SSDI. You can get a copy of of his SSN application with an FOIA request that should have his mother's full name and place of birth.
This webpage has marriage of George and Sylvia, married Choctaw Nation May 7, 1892 George H was age 33 Sylvia Reevis was 18.https://archive.org/stream/oklahomaindiante18901900unit#page...
So she was born about 1874 so look at 1880 census.