Women didn't get the vote until after WWI, so electoral registers will not help. Violet is shown as adopted on both censuses, but there won't be any adoption records as legal adoption did not exist before 1928. If Sarah had her baby in the workhouse, then she might be mentioned in the records - if those records still exist. Any records would probably be at Staffordshire County Archives. They are not available online for Staffordshire. You would need to look through them yourself or find someone who can visit the archives.