... The sea holiday
Was her past, mine is her laughter. Both wry
With the laboured ease of loss.
This quote from the poem holds the answer to the question of whose loss is greater. These lines contain ambiguity intended by the poet that gives the clue to the answer.
The mother's loss is suggested in the description of "wry ... laughter." "Wry" is defined as warped, twisted, sardonic, bitter, scornful, derisive, mocking, cynical. A wry laugh is not a happy laugh. A wry laugh is one that scorns because the expectations of youthful life have not been met: wry laughter scorns the mother's past because the hopes for her present life were not met. For a mother to look an old photo of herself, taken at a happy time, with a wry laugh means that she has lost her hope, her joy, her true laughter, her sense of worth and self. This is a lot to lose. The poem leaves no question that this is what the mother lost because her laughter is "wry" and thus self-mocking: there is only one meaning for wry, and wryness only comes to a person through great disillusionment and bitter personal loss.
When the daughter persona of the poem suggests that her own past loss is the mother's laughter, ambiguity is created. "Past" and "loss" are equated through the word "both" so that since the daughter's past is the mother's laughter, her loss is her mother's laughter. Ambiguity here makes it unclear whether the loss is because the mother stopped genuinely laughing when wry laughter intruded on life or whether the laughter stopped from the mother's death ... or both. Since the poet chooses a photo that she describes with disaffection as "the cardboard," the soundest analysis is that the daughter is speaking of both the loss of genuine heartfelt laughter, replaced as it was by wry laughter, and the loss of laughter pursuant to her mother's death, for she has "been dead nearly as many years" as the girl on the cardboard lived.
The daughter's loss then is two-fold: her loss of the joy of her mother's true laughter and the loss of her mother's life. The mother's loss is two-fold as well: the loss of her belief in her past, which she came to see through cynical eyes, and the loss of hope and joy in her present life. Based upon this analysis of the poem's intentional ambiguity, it seems impossible to say that one or the other had the greater loss. The loss of your own life to yourself, the intrusion of wry bitterness with lost hopes and dreams, is a painful loss. At the same time, a daughter's loss of her experience of her mother's true laughter followed by loss due to her death, is an equally painful loss. The ambiguity of these lines indicates that the poet wants us to understand and mourn the loss that each experiences.
There are two additional points to consider in trying to understand the ambiguity of the losses. (1) The daughter's loss would have been less had the mother not lost her genuine laughter. (2) Both pasts, "sea holiday" and "her laughter," were "wry / With the laboured ease of loss." The personified "sea holiday" was wryly mocking the mother and it experiences its own loss because of their "terribly transient feet," since all three girls have (or will) die like the mother has died.