Hi,
This is a really complicated scenario. It is best to seek an attorney for help on this one.
I see two potential major issues:
1. There is no record of your bio mother from your birth. But there is record of it for your step mother. So establishing mother/child relationship will be a challenge. They may request DNA, but then the question of why your step mother is documented as your birth mother may come into play.
2. If your father never legally divorced, that means the US may see him as having two wives. This is a no-no for the Us government. If you apply for your father, it may be a challenge for him to bring his "current" wife, your stepmother. The US government may see him as still being married to his first wife.
The lack of documentation of your mother (your birth and your father's divorce to her) is going to complicate things drastically. The US government loves documentation to make things easy.
I hope this help explain some potential hurdles your family will face.