In "The Photograph" by Shirley Toulson, what does "Its silence silences" mean?

What sticks out to me about this poem is the overall feeling of solemnity: The mother used to look at the photograph to remind herself of the day it was taken, whereas the daughter looks at it to remind herself of her mother telling the story of the day it was taken. The last four lines seem to be a reflection on the relentlessness of time—in the picture, her mother had been alive for about twelve years; now, her mother has been dead for about twelve years:

Now she’s been dead nearly as many yearsAs that girl lived. And of this circumstanceThere is nothing to say at all,Its silence silences.

It’s one of those things that’s strange to realize, but (in my opinion) the speaker is unable to find the words to explain this feeling further. The silence of the picture and the loss (her mother is not around to laugh at the photograph or to tell the story anymore) causes silence in the speaker and brings about the end of the poem.
The link I’ve included should be helpful in explaining the context!