https://www.uscis.gov/adoption/bringing-your-internationally-adopted-child-to-the-united-states#:~:text=Parents must accrue 2 years,during and after the adoption.
There are two steps for this situation.. your wife must first legally adopt the child. You do not have to adopt. The USCIS step child relationship is acknowledged if the child is your spouses.. But birth of adoption..
SECONDLY .. before you can petition for your adopted step child you must meet the 2 year physical custody requirement as set out in the link above