I am heir in will.
Elder Law Attorney
While the beneficiary of an estate who isn't also a fiduciary doesn't have the right to demand a bank provide the signature card or other records, he may be able to force the estate fiduciary to obtain and disclose those items by showing that they are relevant to administering and settling the estate. This almost surely would require a court application/motion/proceeding and a showing of why the requested items are needed. Another alternative may be to sue the bank and supposed joint owner directly, but you have to have reasonable grounds or you could be subject to sanctions. Essentially, you really need to consult a local trusts and estates lawyer if you think your rights may be violated. Lawrence Friedman, Bridgewater, NJ. Certified as an Elder Law Attorney by the ABA approved National Elder Law Foundation, former Chair NJ State Bar Association Elder and Disabilities Law Section, Member Board of Consultors of NJSBA Real Property, Trusts & Estates Law Section, Vice Chair Special Needs Law Section of National Academy of Elder Law Attorneys, and Master of Laws (L.L.M.) in Taxation from N.Y.U. School of Law.
Unless you are also the executor (personal representative) of the will or decide to sue the executor and go through discovery, you can't. An heir does not have a right to demand this information merely because they are a beneficiary under a will. Your rights relate to receiving your portion of assets provided in the will and to have the executor comply with their duties under the will and state law.
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.
Estate Planning Attorney
Simply being an heir under a will does not give you access to the bank information. The other answer set forth the process for obtaining the information. The only additional question I had was who was the account joint with? If the joint owner survived your parent then you may have to talk to them about getting the information you seek.
I am licensed in Wyoming only. The legal analysis of any situation depends on a variety of factors which cannot be properly represented or accounted for on a web page. The information is intended as general information only, and is not intended to serve as legal advice or as a substitute for legal counsel. If you have a question about a specific factual situation, you should contact an attorney directly.