1. can you please let us know your country of origin because fish antibiotics are not approved by FDA in US for human use and giving them to your sister will be unlawful and moreover without any license.
2. as long as supply of human antibiotics is available, don't go for fish antibiotics.
3. direct evidence is lacking about its safety and suitability for human use, thus talk with your health care professional about the same.
4. most people because they don't have health insurance, or are unable to pay the expenses they just go to pet stores and buy antibiotics meant for fish, as your sister is very poor I think taking fish antibiotic for that matter will be good for her.
5. most importantly tetracycline
for fish is used for EXTERNAL diseases while in humans it is used INTERNALLY.