Hi Colin
So he was RN. I do not know enough about RN WW1 medals to say if they had an index card system like the army. The original National Archives medal rolls which gives B&W cards for six men at a time is supposed to include RND soldiers. But he is not listed. The coloured cards on Ancestry do not. But maybe someone with Ancestry can check that.
The army use the card to summarize the movement of the medals and includes details if the GPO failed to deliver. So you need to find his medal card.
If you do not get help on here I would ask on the Great War Forum if there is another source for RND cards. Or of any other examples of RND soldiers dying in the UK and not getting their medals.
I see that his NoK was his sister. Maybe she did not know that he was entitled. It would have been four years after her brother's death when the medals were sent out. If the navy sent them to her last known address and they were returned, she maybe did not know she had ten years to claim. Was she still at the same address in 1920? I see she was single while he served.
btw...Findmypast have him listed under "died at Sea". They have another transcription -
http://search.findmypast.co.uk/record?id=gbm%2frnd%2f2980 - that says he died in a Nottingham military hospital.
Ken