Yes I would consider her the mother of Delia/Dela as on the census records, but the she could be Brooks mother also but remarried to a Martin, it says related to head of household which would be Brooks, and since both Delia & Isaac say parents born in Louisiana. Have you found all you need on Brooks, to see if his mother was named Sallie/Sally?
I didn't try to find Sallie Martin in 1900 census. Try Sally or Sallie Martin born 1850-52, she should be in 1900, 1880 census maybe in 1870 census also, if she married about age 16. Check census records in Texas and Louisiana.
Do you have the death certificate for Delia? Their marriage record?
if your near a library hat has genealogy records, there are some free search sites you can use at library.