... they looked at us like we were crazy. Adopted as infants, they had no interest in their bio parents, or culture (we tried). Now they're in their late 20's and have still not shown any interest in seeking out their bio parents.

I guess "real" mom and dad is subjective.

The search for our biological family isn't really about the family, but finding ourselves. Adoptees do it at their own pace, in their own time. The more that you feel that they have no feelings about their biological family is likely an indication of just how much they don't tell you.

... and the offer remains open.

best wishes