Thank you so much for offering your help! I've added some more details. Are they helpful? Please let me know, thanks a lot!
Also, in this situation, after the marriage, I will not be considered as "adopted" by the US citizen step father, right? Thank you!
Yes, the additional information helped. Please see my analysis below. No, you will not be considered adopted by your step-father. Should you decide that you want to be adopted by him, there is a separate process that goes through state court. You would have to consult a family attorney in the state where you will all be residents to determine if it's possible (given your age) .