If they have a suspect in custody, a reasonable suspect, and she did receive serious injuries, then her account is most likely true.
There are psychopaths who kill people, even family members, injure themselves, and make up a story about an attacker. Charles Stuart and the Lululemon killer come to mind.
I doubt premeditation by a 12 year old rape victim - a victim who watched her brother murdered. The only motive I see in this case is the motive of a terrified child not wanting to be murdered at the hands of her rapist if she ‘snitched’.
What rational motive could there be for a 12 year old other than what I’ve said? Large sums of money left by her wealthy eight year old brother perhaps?