It is most unlikely that your sister was cured by Lamivudine
in just a few months UNLESS she was infected as an adult, had acute
Hepatitis B, and had a full recovery as most adults do. Lamivudine was just incidental. Lamivudine is no longer used to treat Hepatitis B in most countries, so the opinion here is that don't use Lamivudine.
At the moment, there is no quick cure for chronic Hepatitis B.
Tenofovir and Pegylated Interferon are both first line drugs for Hepatitis B.