Since the 2019 reform there are two ways into cassation, and they behave completely differently.
Continuous cassation applies within six months of the verdict entering into legal force. The complaint is filed through the trial court and the cassation court is obliged to hear it on the merits. There is no filtering judge to get past.
Selective cassation applies after those six months. The complaint goes directly to the cassation court, where a single judge decides whether to refer it to a hearing at all. Most are refused at this stage, in a short reasoned decision. There is no time limit on filing, but the practical prospects are far lower.
The difference is stark enough that the six-month deadline should be treated as the real one. If your relative's verdict entered into force recently, this is the point to act.