Mom’s Will/Trust

The only way to compel someone to disclose documents is in a lawsuit. If you believe that he is violating the terms of a will or trust under which you think you are, or at least likely to be, beneficiairies, you could bring a lawsuit in which you seek to compel production of those documents and could also, in the course of the lawsuit, use the legal tools or mechanisms of "discovery" (e.g. written questions or interrogatories, document production requests, subpoenas) to get information or documentation. If it turns out he has kept back money, assets, etc. from you, you could then seek a court order requiring him to turn over to you whatever should have gone to you, or seeking other monetary compensation. Again, though, you won't know if you are entitled to anything until after you file the lawsuit, since you need the lawsuit (which would be against your mother's estate; her trust if there is one; the personal representative of her estate; any trustee(s); and your stepfather) to find out if there was anything coming to you.
Therefore, you would have to sue before knowing for a fact that there is a reason or grounds to sue, in order to see these materials. And if it turns out that the lawsuit is baseless or unwarranted (e.g. there is no will or trust; or there is, but you are not beneficiaries), it is possible you could be countersued or face sactions (e.g. have to pay his legal fees) for frivolous litigation.
Also, this type of claim is substantially more procedurally complex than, say, a small claims case, and you are strongly advised to retain an attorney to help you--but you'd most likely have to pay your own legal fees, even if you win (and you'd certainly have to pay them if you lost).Therefore, while you can do this, it is not something lightly done.