It is also possible that there is no specific feeding issue, and the girl is receiving the bottle out of habit. For some autistic children this may impair further their ability to interact with their surrounding environment, but for others it may soothe them enough to allow them to interact in a more meaningful way. There is no way for me (or you) to know what the circumstances of her bottle use are without speaking directly with her parents. You certainly could ask your sister in a non-judgmental way, why it is that her daughter still requires a bottle. You must then be willing to listen to her concerns and opinions, and help support her in whatever way may be useful. Parenting an autistic child can be overwhelming, and most families need some help. You can get more information about autism and related disorders (known as pervasive developmental disorder or PDD) from reputable websites such as the Autism Society of America or the National Institute of Mental Health.
Autistic Four-Year-Old Still Using Bottle
Pediatrics Expert Advice from Shari Nethersole, M.D.