My deceased father wrote three novels and my sister and I now possess the manuscripts for two. What copyright law applies if we were to publish these works? He has been dead for thirteen years and left his wife, our stepmother as his sole heir, though the manuscripts are not mentioned in the Will. Would they be copyright to us, to him, to her? Does possession of the physical manuscripts matter?
In the US, the right to publish is vested in the copyright holder, who is initially the author. That right can be transferred for example by a transfer agreement, and it can be inherited just as other property can be inherited. Under the terms of the will, it is most likely that the copyright was transferred to the spouse even if the will didn not say "including all copyright". There is a small chance that it wasn't disposed of if the wording of the will is restrictive enough (for example "I bequeath all real estate and tangible property to my wife", which doesn't include intellectual property). If so, that would definitively require the assistance of an attorney, and a court proceeding to dispose of the copyright. If there was no provision under the will for copyright, then the copyright could be divided accounting to the rules of succession of your state, however, the courts would want to be persuaded that it was not his intent that his wife receive the copyright as well. Possession of the physical manuscript is largely irrelevant – it does not give the manuscript-holder the right to override copyright law, although if you are in lawful possession of the only copy of the manuscript, you may be able to thwart plans to publish.
Publishers generally require decent evidence that the person submitting the manuscript does legally hold copyright, when a work is submitted by someone other than the author.