I found my mom and stepdads divorce paperwork after telling my sister that there relationship would be fine
Have you told your sister anything else?
Not yet.
That seems to be something between your mom and stepdad. When the time comes, let them tell you.
I can't just let them do this to my sister.
Divorce is a very delicate situation that should be handled by the parents, not you. This is none of your business, really.