Just seen previous posts by yourself in which you say she possibly married a Jackson. Where does Dawson fit into the picture?
I have done the following with no results:Masters PortalZimbabwe / South African records on familysearchEggsa tombstonesThe archives - under Jackson, Dawson as well as just Nellie LouisaMakes me think that she did not die in South Africa.
If her sister's children think she was a Jackson what other details do they know about her. Would they know where she lived, whether she had children etc.?
Have you looked into Charles Donahue? Maybe his paperwork would show whether their were children from that marriage. Her parents wills / estate files might show other information as well.