Re: False sexual abuse claims by autistic child
Your sister seems to be verbal enough to have a decent discussion with... Have you tried talking to her about who first "informed" her about what had "happened" to her? If so, you may be able to find out who coached her, if she was coached.
Can she write? Is her written communication better than her speech? It might help to have her write down what happened, et cetera.
It is important to consider the possibility that she is telling the truth, however. She may be pointing to the wrong person--might someone other than her stepfather have done this?
Have you asked the teachers for a copy of their sex ed curriculum? See if you can match that with what she's saying--what did they teach her about abuse, about the acts she is recounting? What, exactly, did she see her brother do?
Regarding autism and sex ed: I have Asperger Syndrome, which is high-functioning autism without language delay; so my brain may have some similarities to your sister's. I learned about sex at the age of nine, when I read my mom's "women's health" books. Since then I have learned about the mechanics of sex in most of its forms, though I am still a virgin. Generally, my attitude towards sex is the same as towards other bodily functions... I attach very little significance to it, have little desire to have sex, and identify as asexual. I have been known to make casual comments about sex, only to find out that what I said was taboo and embarassing to those around me.
Social expectations are not always clear to me, and they may not be clear to your sister. Does she tell stories about other things which did not happen? If she does, this may be yet another story. How do you usually ask whether something is a story or really happened? Can you use this method to communicate with her?
Many autistic people, unlike me, do want to have sex, of course; your sister, at the brink of puberty, may be experiencing such desires. If she tells stories about things she wishes would happen, then she might do so if she wishes she could have sex. She may not have taboos about incest in the same way most people do; autism often stands in the way of becoming a member of the dominant culture, so if she does not have these taboos, it may seem natural that, if she wants to tell a story about sex, she picks one of the most significant males in her life--her stepfather.
Did your sister come up to you and tell her story about sexual abuse; or did your stepmother and father initiate this discussion?...
It might be best to get an uninvolved specialist to talk to your sister. This way she has no emotional connections to the person, and is more likely not to be influenced by expectations. It may take a while for communication to be established between her and a new person, though.