Hi, A.! I know you wrote your message awhile ago, but I am just now getting to read some the messages that were sent the past few days!!!
I am a mother of two boys who are almost three and the other just turned four. Both of my sons are autistic. If you are concerned about Kaydan, it wouldn't hurt to get him evaluated, if for no other reason than to rule autism out and give you some peace of mind! If his behavior is typical for a child his age, then at least you can have peace of mind and you can tell your sister-in-law that what she is seeing is typical and not to worry about it. There are some behaviors that are "signs" of autism, but that doesn't mean that every child who exhibits that specific behavior is autistic.
On the other hand, if your sis-in-law is accurate in her concerns about Kaydan's behavior, you don't want to waste time wondering "is he or isn't he?" Call your pediatrician or the birth to three/early childhood program in your area, and they can help you to determine whether he is on the autism spectrum or not --or whether he even needs services or not.
One last thing: because, it seems, autism is more and more being diagnosed in children, you, your family, and friends will be seeing more on TV and hearing more about it in the news. Don't let what you hear scare you. Keep in mind that a diagnosis cannot be made by watching a "kiddo" on TV exhibit certain behavior and because your child does the same thing, that automatically means YOUR child is autistic. It doesn't mean that. Like I mentioned earlier, if you are concerned, and you feel there may be some delays or you are questioning certain behaviors you see in your child, ask your pediatrician. What may be an extreme behavior in an autistic child, may be typical behavior and a passing phase in a typically developing child. I hope this helps some!
P.S. I am not a doctor or a professional (outside of having my own two autistic boys!), but typically, autism is not able to be diagnosed until around 18 months. Eleven months is a little young to really know for sure! Still, talk to your doctor for your own peace of mind!