Tito Mancini

Outwardly, Gianni looked unfazed and his face betrayed no emotion when Enzo threatened to report to their grandfather about Rosa but inwardly, his heartbeat was a little faster than normal. He was scared for his little sister whose harmless act might be seen as an act of betrayal by his grandfather

His grandfather was infamous for his violent temper and unforgiven nature and there's no question what he would do if he finds out about Rosa's mistake. Gianni knew if his grandfather didn't kill Rosa outrightly, he might decide to punish her the way the Mancini always punishes errant female members of the familia who had committed what his grandfather always tagged as a grave sin against the familia. 

Gianni decided in his heart right there that he would rather die than have his sister sent to a brothel to be a prostitute where she would be turned into a heroin addict so the pimp assigned to her would be able to easily control and influence her.