Found this on USCIS Website:

For example: You marry someone with a child. The child will
usually qualify as your stepchild if he or she was unmarried and
under 18 years of age at the time of your marriage. In this example,
you are required to file two petitions: one petition for your wife and
another for the child.

Looks like your step parent can file for you, but it looks like you might have to be under 21 , so I would recommend talking to a lawyer as soon as possible.

Also, I think people with DACA have been able to fix their EWIs through AP.

But honestly, looks like you need to talk to a lawyer to get all of this figured out before you are 21.