The Reverse of the coin shows the three sisters as personifications of key Roman concepts: Agrippina as 'Securitas'; Iulia as 'Fortuna'; and Drusilla herself as 'Concordia'. 
Drusilla's relationship with Gaius was allegedly of an incestuous nature. Although married to Marcus Lepidus, Dio (59.11.1) states that "she also slept with Gaius". Suetonius (Gaius 24.1) extends this sexual relationship to all three of his sisters.
However, given that incest was an accusation frequently levelled at tyrants (see Tacitus, Annals 14.2, on the allegation of incest between Nero and his mother, Agrippina), we may perhaps question the validity of such claims.
Josephus (Jewish Antiquities 19.203-204) also makes this claim, and this - the most contemporary of the historical accounts to survive - was written under the ensuing Flavian dynasty, who did all they could to break with the worst of the Julio-Claudians, i.e. Gaius and Nero.
However, Gaius was certainly distraught at the death of Drusilla, "bestowing upon her all the honours that had been given to Livia" (Dio 59.11.2), including deifying her and having a golden statue of her set up in the Senate House. She also received the title 'Panthea'.
That this course of action was followed is highlighted by two surviving inscriptions, one from Mytilene which addresses Drusilla as the "new Aphrodite" (IGRRP IV 78b); and a second from Tibur (modern Tivoli) where she is "Diva Drusilla" (ILS 196; CIL XIV 3576).
