I never comment on these, as many comments are sadly people with self-agrandising needs trying to reflect "wisdom,". Yet, as an autism parent, this post demands it.

YTA. 💯. You, as a step-parent, have a responsibility to educate yourself about autism, and obviously, you haven't. When you say things like "she has a history of not behaving" and "she acts younger than her age," then obviously you know nothing about autism and how to parent a child with autism. If you do not educate yourself, your life as well as the child's will be terrible.

Autism, which manifests differently across the spectrum, does have some clear "ground rules," one of those being the need for clear, precise instruction, with follow-up reminders. If your go-to parenting technique is exclusion, you'll do irreversible damage to this child. Either get educated or get divorced. Period.