He is referring to Gaius Cassius, Marcus Junius Brutus, and Decimus Junius Brutus.
Marcus Brutus (whose father had been killed by Pompey, but he sided with the Pompeians in the Civil War of 49 B.C. anyway) was the son of a Servilia who had a long affair with Caesar, but that started after Marcus was born (so Caesar wasn't his real father).
Marcus and Decimus may have been related but I don't think we know how closely--I would think that if they were close kin some ancient source might have noticed the fact.
Thank you. Interesting to find this out.
I know that these early historians were not as concerned with detailing things as we would think, today.
Hence the need not to rely on one historian for all things. Good information. Appreciate it:)