Definitely hire a lawyer

Laws, especially custody laws, vary so greatly by state that a lawyer in your state will best be able to help you determine your BFs rights in this case. I'm suer you've discussed this, but are you and your BF planning to get married after your divorces are finalized. Again, the lawyer will know how that may help you case in your state, but I know where I am being married to my DH looks significantly better to the court and allows us more rights where SD is involved.
If you cannot have any legal rights to his oldest, as hard as it is, it's still going to be best to have something in writing. It sounds like his divorce is still fairly new? That would mean emotions are high and logic and the children's best interest might not be in his ex's mind right now. I would give it time. If he continues to be actively involved in his biological daughters life and her sister continues to show interest in seeing him, I would imagine it won't take too long for her mother to realize that it truly is in her best interest to be allowed access to someone who played such a huge role in raising her.