It’s a horrific case, which deserves a thread of its own.
I don’t know the French penal system at all but those sentences seem very lenient to me.
Apparently Dominique has the maximum sentence possible which is 20 years. Given he is 72, it's unlikely that he will leave prison alive.
There is a comment from Marechal's lawyer that with a 12 year sentence, Marechal could be back out of prison in 3 years, so it appears possible that with good behaviour and rehabilitation the French system is geared toward home release rather than just keeping people behind bars.
It does say in the feed that the sentences are generally a couple of years lighter than the prosecution were expecting.