About Tiberius Gemellus
Tiberius Julius Caesar Nero Gemellus, known as Tiberius Gemellus (10 October AD 19–AD 37 or 38) was the son of Drusus and Livilla, the grandson of the Emperor Tiberius, and the cousin of the Emperor Caligula. Gemellus is a nickname meaning "the twin". His twin brother, Tiberius Claudius Caesar Germanicus II Gemellus, died in infancy in 23 AD.
Gemellus' father Drusus died mysteriously when Gemellus was only four. It is believed that Drusus died at the hands of the Praetorian Prefect, Lucius Aelius Sejanus. His mother Livilla was put to death because she had been plotting with Sejanus to overthrow Tiberius, and also because she may have worked with Sejanus to poison her husband.
Little is known about Gemellus' life, as he was largely ignored by most of the Imperial family, so much so that one of the major landmarks of his youth, the toga virilis, wasn't celebrated until he was eighteen. The normal age to celebrate this was fourteen years.
At the age of twelve Gemellus was summoned to the island of Capri where Tiberius lived, along with his cousin Caligula. Tiberius made both Caligula and Gemellus joint-heirs, but it was clear that Tiberius favored Caligula over his own grandson. Livilla had been Sejanus' lover for a number of years before their deaths, and many believed that both Gemelli were really Sejanus's sons.
Tiberius died March 16, 37 AD, and Caligula became Emperor. Caligula made Gemellus his adopted son not long afterwards, but ordered him killed in late 37 AD or early 38 AD for allegedly plotting against Caligula while he was ill.
Little has been written about Gemellus. Most of the information we know about him has been connected to material about Caligula.
English Wikipedia: Tiberius Gemellus