I don't know much about this topic but I just want to chime in to ask if formal adoption is even necessary. If your only aim is for her to be eligible for citizenship, doesn't she just need to be physically present in the U.S. and in the custody of her mom, your wife? I was under the impression that whether or not you formally adopt her has no bearing on that, but I leave it to the wisdom of the forum. If you want to do it for sentimental reasons, go ahead and research the possibility, but from an immigration perspective I think as the stepdad you're all set. Curiously following along to see what others say and willing to stand corrected.