What should I do if my stepmom kept my green card and she doesn't want to give it to me?

Sorry to hear about the problematic family relationships you mentioned. It would be possible for you to file an application to receive a replacement "Green Card." Additionally, there may be steps to consider in trying to persuade your stepmother to release your "Green Card."

You need to speak with a lawyer. If you have a permanent green card, without a condition, you can simply file a form to replace it.