Copyright endures for 70 years beyond the death of the creator regardless of who subsequently inherits the works.
If he will do everything in his estate to your grandmother but made no specific mention of these artworks, then the copyright belongs to your grandmother.
I am assuming that your grandmother has now passed away.
If she then does the same and leaves everything to you and your cousins with no mention of the artwork specifically, then you and your cousins own the artworks in the same proportions that you inherited everything else from her estate.
You can of course do a deal with your cousins whereby they get a royalty for allowing you to use these items or they may not be bothered.
Alternatively, you can agreed between you, you could agree to buy out their interest in the artworks or divide them between you, whatever you can agree. They are more likely to agree if they think that there is no value in them.
Can I clarify anything else for you?
I’m happy to answer any specific points arising from this.
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