Rosalia (Rosetta) Messina Denaro, 67, was detained in Sicily on mafia membership charges, Carabinieri police said, releasing a 57-page arrest warrant.
Her brother, Matteo Messina Denaro, 60, was Italy's most wanted man until his arrest on Jan. 16, after 30 years on the run.
Italian authorities had already said that Messina Denaro was snared by his failing health, but on Friday they revealed the source of the key tip.
'BIG STRAWBERRY'While on the run, Messina Denaro followed mafia tradition in communicating with relatives and affiliates via "pizzini", small pieces of paper sometimes written in code.
Rosalia's husband, Filippo Guttadauro, is another convicted Sicilian mobster, while her daughter Lorenza is Matteo Messina Denaro's defence lawyer.