Short answers, pulled from the story.
Lucius Cornelius Cinna entered the world before 130 BC into a patrician family that lacked recent distinction. The historian Theodor Mommsen once argued the Cornelii Cinnae were plebeians, yet modern scholars largely view them as true patricians.
Cinna married Annia and they produced three children together: two daughters and one son who shared his name. His eldest daughter wed Gaius Julius Caesar during Cinna's second consulship in 86 BC while his younger daughter married Gnaeus Domitius Ahenobarbus.
The Senate rejected Sulla's candidate Publius Servilius Vatia Isauricus and elected Cinna with Gnaeus Octavius instead. This election occurred during deep revulsion over Sulla's march on Rome when Octavius took an oath seriously while Cinna did not.
Appian reports Cinna attempted to impose discipline which culminated in mutiny and his death among soldiers waiting at Ancona. Plutarch offers alternative story about soldiers fearing Cinna assassinated young Pompey though this lacks credibility.
The Samnites demanded citizenship and release of prisoners when Metellus negotiated peace. Cinna offered these concessions and gained the Samnites as allies before storming Rome through gates opened by a defected military tribune.