Hello and thank you for writing to JustAsk!

I know that this is a tricky question because you don't want to "rock the boat" with your twelve year old, however you wish for an addition to your family.  I would think that not all children with autism respond the same to a new sibling. Thus, you need to proceed slowly and take a few cues from how your step-son reacts to the introduction of other people and objects in his environment.

You may wish to have your step-son interact on a regular basis with babies, toddler and preschool children in small doses.  (Perhaps children of friends).  I would start slowly.  A quiet place for an interaction of a short time span.  Then increase the  amount of time per interaction, as well as switching the kinds of interactions.  You may wish to include a meal or play time. He may need to see you taking care of a young child outside of your home and then in your home.  Maybe if he is gentle and can sit still long enough to hold a baby that is not a newborn with close adult supervision for a short while this may assist him in the tactile portion of having a new child enter the home. The key is to have your son acclimate to the sounds, smells and visual stimuli that a new baby, toddler and preschooler can bring to a home.  

Also, I would not leave a baby unattended in the same area as your step-son. Tweens in general can be very clumsy as their bodies develop and also forgetful.  Thus, supervision would be warranted.

Good luck to you and your family!  

Louise Masin Sattler, NCSP
Nationally Certified School Psychologist
Owner of Signing Families
http://www.Signingfamilies.com

Book recommendation:  Siblings Without Rivalry by Adele Faber, et al.