Amanda Lee is the adopted sister of NCIS Special Agent Michelle Lee.
When their parents died, Michelle took on a more motherly role for Amanda, raising her as a daughter while also disguising the fact that Amanda was really her sister, not her daughter.
Amanda was kidnapped by The Weatherman (whose real name was Ted Bankston) and he used her kidnapping as leverage to make Michelle attempt to steal military secrets which she did, even committing murder to do so. NCIS later discovered what was going on and got involved in an attempt to save Amanda and stop Bankston.
When Michelle and Bankston were both killed after Leroy Jethro Gibbs shot both of them, Gibbs later gave Amanda Michelle's NCIS badge.