Interesting.
Isn't there a coroner report of some sort? Probably very difficult to establish but; if it could be established that the girls died on impact when the Fiat Punto flipped the Ford Ka over, Carrera could not be convicted of culpable homicide, as the girls were already dead. He could only then be fined for reckless driving.
But in all probability it all happened very fast, and it probably can't be established which of the impacts killed the girls. They are viewing the incident as a whole. Hence he is dragged into the culpable homicide-case.
Medical science and technology could surely inform us of which of the hits were the cause of death, no?
You examine the two bodies, analyze where their heaviest damages are - the ones that had to have caused death - you compare it with where they were, how they sat in the car and with the first and second impacts of the cars that crashed into the Ford Ka. Provided that all facts on these matters are gathered, of course.
Although difficult, it should be possible to establish from that whether the heaviest injuries causing death had to have come from the Fiat Punto or Carrera's Mercedes.
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Final point: I'm not disputing Carrera's recklessness, that's an entirely different topic. Recklessness on his part may or may not have been present. I know too little facts to have an informed opinion on that. But what I'm trying to illustrate here is that recklessness isn't enough, the recklessness has to be linked to death in order to be convicted for culpable homicide. Classic "cause/effect"-considerations will come into play, considerations we all know from other fields of science. Although the cause/effect-consideration in Criminal Law is based on legal principles (normative) rather than the ones from the natural sciences (descriptive), you still have to be able to say that Carrera's action was a condition (conditio sine qua non) for their death.
You have to be able to link Carrera's culpa to their death, like that.
Looking at the facts JuveJay posted, I think serious doubt could be cast on whether such a link exists.