First, if your sister lived in New Jersey, then you need to direct your question to a NJ probate attorney.
Regarding the will, this was most unfortunate that your sister did not store the will in a fire-proof safe or other secure location. Usually, other attorneys may take over for attorneys who died - if the lawyer practiced in a law firm that is still in business, you can check with the firm as they have to store the client's files. The problem is that lawyers may or may not keep signed copies of wills for clients. I know that as a matter of practice, I do not. I keep only drafts of the wills and since they are not signed they cannot be probated.
However, you need to check with a NJ probate attorney who practices in the county where any estate would be filed for your sister. Failing that, contact the clerk of the Surrogate's Court (that is the probate court) and ask them how a will can be probated if it was accidentally destroyed by fire. Some states may have a process for proving the contents of a will (that is where the copy may help) in the event of destruction by some force (like fire, earthquake, flood, tornado or hurricane).
If all else fails and the will cannot be proved, then your sister's assets will be disposed of under the NJ intestacy laws. In most states, the order of preference is surviving spouse, if any, and children or lineal descendants (grandchildren, great-grandchildren etc.). If there are no spouse, children or lineal descendants, and if both her parents are deceased, then you and any other siblings would inherit.
I would strongly encourage you or any interested sibling to meet with a probate attorney. If the sister's home was destroyed by fire, there is going to be an insurance check headed your way and it will have to be handled correctly.
I would recommend contacting the bar association for the county where her lawyer practiced. They may be able to tell you which lawyer took over his practice when he retired or passed away. That attorney may have possession of the file.
It is also possible that the will is on file at the courthouse, depending on New Jersey procedure, with which I am not familiar.
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First I would like to start by expressing my deepest sympathy for your loss. Second, I can tell you that most likely the attorney did not have a copy of your sister's will. Under New Jersey law, duplicate wills are invalid unless both duplicates are produced. As a result I do not know of any attorneys in New Jersey who do this. The attorney's file might have notes from his meeting with her stating who she wanted to be the Executor and other wishes. This is not legally binding but may give you insight into what she wanted. Also, if a person dies without a will, the state imposes a structure to distribute their assets. I hope this was helpful during this most difficult period. I wish your family luck.
If you do find a copy, you could file a complaint with the court requesting that they admit to probate a copy of the will. This has been allowed in NJ. Without a will, the estate assets will pass to the heirs at law.