As a 25 year-old, you are not an "immediate relative" of either your father or stepmother, nor are you of an age to be included as a "derivative" on any application your stepmother may have filed for your father. It is not clear whether you currently have legal status in the United States (e.g. are you studying on an F1 student visa?) If you do not have status now, there is no way you could get a green card through your father or stepmother without leaving the country, which may subject you to a 10-year bar from returning. The situation as you describe it raises many questions. You really should have an in-depth consultation with an immigration attorney before filing anything.