Why didn't Marie-Loup simply tell the truth?

***SPOILERS***

Why did Marie-Loup feel she had to sacrifice herself by allowing everyone to believe she killed Xavier? The truth is revealed at the end that little France killed Xavier in self-defense as he was about to rape her in a drunken stupor. What court would possibly convict an 11 year-old of murder when it was clearly self-defense? It didn't seem like Xavier had any political pull at the time to engender such an extreme response from Marie-Loup (sacrificing herself for her daughter).