Well, you have to look up the actual adoption documents yourself and check for certain who exactly issued them.
However, at the end it may not matter whether the adoption was foreign or domestic.
For some adoption-related visas (such as IR-4 and IH-4) a re-adoption in the U.S. is required in order for the child to derive citizenship under the CCA. However, it appears (if I am reading it right) that for CR-2 a domestic re-adoption is not required.
See pp. 26-27 in
http://www.state.gov/documents/organization/86759.pdf :
"CR-2 - The child is the admitted conditionally into the United States as the stepchild of a U.S. citizen who has been married to the alien parent for less than 2 years. The conditional category is valid for 2 years and may be adjusted to an IR-2 at the end of the two years. It is a conditional category because of the possibility of marriage fraud.
A child admitted as a CR-2 acquires U.S. citizenship automatically as soon as he or she meets the statutory requirements of INA 320, as amended.
Documentary Requirements
If the child is admitted as a CR-2 - the parent's marriage certificate and a copy of a full and final
foreign or domestic adoption decree by a U.S. citizen. In order for a child in this category to acquire U.S. citizenship, the adoption must have taken place by age 16 and the child must have resided with and in the custody of the U.S. citizen for 2 years. The date which citizenship is acquired could be (1) the two-year anniversary of the marriage which should coincide with the 2 year residence requirement;
or (2) the date of the full and final domestic adoption.
Passport specialists at domestic passport agencies and centers and consular
officers abroad must verify the date of marriage to determine if the 2-year requirements (length of marriage and legal and physical custody) have been met. If one or both have not been met, the application must be denied. However, the applicant should be advised that citizenship may be acquired as soon as the 2-year requirements are met. (Acquisition will depend on whether or not the child is still in the legal and physical custody of the U.S. citizen on the day the 2-year requirement is met.)"
However, you should certainly clear that out with a lawyer and also check what, if any, documents you'd need to submit showing that for at least two years after your enter in the CR-2 status your mom and your step-dad remained married and that you resided with them in the U.S.