I am a step-mom to a now 13-year-old girl. My DH and I have 6 daughters between us (18, 16, 14, 13, 11, 9). Our 13-year-old was diagnosed with Asperger's, high functioning autism, and ADHD before I entered the family. I have been in her life for 5 years. My background is in Early Development and Education, and I entered this marriage with full knowledge of two things:
1. When in doubt, LOVE MORE.
2. Children should not make/break a marriage.
My advice to any "bonus"-parents (AKA step-parents) to children on the spectrum:
For YOUR OWN SANITY:
1. Acknowledge their actual developmental delays:
--> by doing so, you can breathe a bit easier and avoid frustration
2. Check out SUPER-NANNY episodes.
--> children on the spectrum respond well to consistency. Posted rules. Posted Rewards. Posted repercussions. Refer to the posted info as often as necessary so it takes the onus off of you as the "evil-step-parent."
3. TRY (not saying it's possible!!!) to COLLABORATE with the other households involved in raising your step-child.
--> Nothing is more frustrating for me than getting our daughter into a wonderful pattern of self-efficacy, self-esteem, self-empowerment over a long weekend with her, only to have her visit her other family members and return home regressed and "learned-helpless," even to the point of her being unintentionally manipulative.
I can't say it's possible to get everyone of your step-child's caregivers on the same page, but it doesn't hurt to try. We meet every other month to check on status, development, issues, etc. Even so.... doesn't mean they follow through on their end in a consistent manner.
4. RE-ENTRY. As children of divorced households travel back and forth between homes, rules, expectations, we make it a point to remind ALL of our children that they are at THIS HOUSE NOW. And give them about an hour to decompress before asking them to fall into chores, etc.
Hope this helps!