Interesting, and I hope for their sake not too hasty. The final report may well not support them.
From the BBC article, it appears the lawsuit was filed just prior to the one year anniversary of the crash. The timing causes me to wonder whether, for legal reasons, the family had to file a lawsuit now as a protective measure just to preserve any claims they might have (and thus could not await the final report).
Statutes of limitation prohibit filing a claim more than a specified period of time after an event. In some jurisdictions, the time allowed is relatively short. The duration varies depending upon such factors as the country (or, as in the United States, the particular state) and the type of claim (e.g., different time limits may apply to claims for negligence, breach of contract, defamation, fraud, and wrongful death).
I don't know the law of Ireland (or other nations whose law potentially might be applicable depending upon the claim asserted). For the moment I'm inclined to give the family the benefit of the doubt regarding their motives until shown otherwise.
Though your point certainly is well taken, as are some of the concerns mentioned by other posters.