My future step-daugter has signs of autism. what should i do?

I became concerned about the development of my future step-daughter when she was around 6 months old. There were some things that my son, who is two months older, was doing that she wasn't. Her name is Kiera and she is now 15 months old. My son is 13 months old, and my son was crawling, babbling, and doing all the normal baby things while she was still not doing them. She also showed signs of being unresponsive to people or and would focus intently on one item to the exclusion of others for long periods of time. She fails to respond to their name and often avoid eye contact with other people. She also uses repetitive movements such as rocking and twirling, and sometimes uses self-abusive behavior such as biting or head-banging when she is upset about something. My son has started talking before and it has taken her over a month to become interested in copying some of the sounds that he was making. She can now say da-da, but will only make sing-song sounds. She did not start to smile until she turned 12 months. She is extra-sensitive to changes in a person's change in voice tone. She will cry if someone changes their tone of voice if they are saying something to her brother, eventhough her brother and that person are in another room. She is also obsessed with pointing at things or touching things with either her thumb or middle finger, and it will be with either hand. She will also concentrate for five minutes, or longer, on touching her middle finger of one hand to the thumb of the other hand and twisting the fingers back and forth. Could these be the signs of autism or another form of mental disabilities?