Brother, not twin
Drusus Caesar was a younger son of Germanicus and Agrippina the Elder. His career was closely paired with that of Nero Caesar, but the brothers were born in different years and were not twins.
Imprisonment under Tiberius
After Nero’s removal, accusations and court politics also brought down Drusus. He was confined beneath the imperial palace and died in 33 CE after severe deprivation. His younger brother Caligula was the only adult son of Germanicus to survive Tiberius’s reign.
Historical sources
Tacitus, Annals 4.4; 6.23–24
Drusus’s public position, imprisonment, and death.
Suetonius, Tiberius 54; Caligula 7
The three surviving sons of Germanicus and the fate of the two elder brothers.