In the poem "The First Snow Fall" by James Lowell, how do we know what happened to the speaker?

The speaker's daughter has died, she must have died recently, and been very young.  We know because he describes the beauty of the snowfall and talks about "a little headstone" (line 18), our "first great sorrow"(line 27), and the fact that it is the first snowfall (title) that hides "the scar that renew(s) our woe" (line 32).  In the latter part of the poem the speaker is addressing another daughter, "our little Mabel" (line 21). It is her sister who has died and is now "folded close under deepening snow" (lines 38-9).