Standard disclaimer for legal questions: I am not a lawyer, and therefore cannot offer any legal advice, other than to recommend you seek qualified legal advice from a lawyer. Don't rely on legal advice from random strangers on the Internet.
In the US (and Canada I believe), unless otherwise transferred or granted by explicit license or contract, copyright belongs to the person who took the picture, regardless of ownership of the camera or the media on which the image was recorded.
This means that if you hand your camera over to a stranger to take a picture of you in front of a landmark, then technically, the stranger owns the copyright. Of course, this is almost impossible to enforce, and I can't imagine the copyright office bothering to get involved in a putative dispute such as that.
This also means that if you let a monkey hold your camera and it accidentally manages to take its own selfie, you do not own the copyright of that image. (It can't have any copyright assigned, because the copyright office won't register rights to non-humans).
Under the Berne Convention for the Protection of Literary and Artistic Works, signatory countries agree that copyright is established the moment a work is "fixed" (i.e., you possess copyright to a photo the moment you take the image), and that countries agree to honor the copyright laws of other signing countries where international issues may arise.
In this specific case in question, where the works were created by a Cuban national in Cuba and the rights are being sought to transfer a license or ownership to the U.S., this situation certainly falls under Berne Convention issues.
The following two-part article series by Nicole Martinez writing for Art Law Journal covers transference of derivative rights from a Cuba artist to an artist in the U.S.:
Briefly, in the articles, a Cuban author wrote very popular children's books. The author's friend Miguel, while living in the United States, adapted the books into a screenplay, and sought to have the screenplay produced into a film. As the screenplay is a derivative work, before such a film could be made, studios needed to secure rights to the adaptation. Whether, and how, the rights could be granted or transferred is the subject of Martinez's two-part articles.
Quoting from part II, Martinez writes,
Does a Cuban national have unilateral authority to grant a license for copyright, when the licensee is a foreign citizen?
As we discussed in Part I of our series, Miguel and his friend would not be able to enter into a contract for the use of the copyright under the existing trade embargo between the U.S. and Cuba. But they may be able to contract under Obama’s new regulations, if Miguel and his friend are able to prove that importing the copyrighted work into the U.S. “greatly enhance the free flow of ideas between Cuba and the United States.”
Unfortunately, Cuba has laws in place that restrict the free usage of copyrights in other states. Under Article 42 of the Cuban copyright law, a Cuban author may only grant the transfer or use of his work abroad through special permission by the Cuban government. Once again, Miguel and his friend would need to have their contract for the use of the copyright approved before they can move forward, and the Cuban government would have the unilateral right to refuse that the work be used within the United States.
And since Cuba is a subscribing nation to the Berne Convention, that means that the U.S. will need to apply Cuba’s copyright laws to the contract between Miguel and his friend. This effectively means that unless the Cuban government approves the use of the copyright abroad, Miguel will not be able to adapt the work for U.S. cinema.
I highly recommend reading both articles to get a better understanding of the issues in play: the Berne Convention; Cuban copyright law; longstanding U.S. trade embargo against Cuba; lifting of certain embargo restrictions under the Obama administration. And of course, since the articles were written, there are new developments in the U.S.–Cuba relations since Obama's administration.
These articles are of course not definitive on the subject, and certainly might not encapsulate the situation regarding the current question (mainly, transferring of photo copyrights amongst family members across international borders). But they provide an interesting basis to begin understanding some of the issues involved.