My brother's 16-month-old frequently slams her head into the floor when she doesn't get her way. This is becoming quite concerning as she has bruised her forehead on occasion. She will do this behavior with anyone who watches her. She has a half-brother with autism, so this is also a concern. Her development seems to be on track otherwise.
I can see why you and your brother would be very sensitive to this possibility. The hallmarks of autism in young children are abnormal or impaired social skills and communication, combined with having very few interests and activities. Autism also seems to run in families.
However, boys with autism outnumber girls by about five to one, with girls more likely to be profoundly retarded as well. While some autistic children engage in head banging or other self-destructive behaviors, so do many other children this age.
I would not leap to the conclusion that this girl is autistic, especially if she's establishing good social skills for someone her age. In other words, does she sometimes smile when she sees people she knows? Does she respond when you call her name? Does she point to things that she wants?
Still, it would be a good idea to discuss your concerns with her pediatrician. Remember that despite the old wives' tales, we now know that autism has nothing to do with whether one has "good" parents. It's a biological problem.