Alliteration of the initial "s" sound is also known as sibiliance, but the alliteration in this poem that you have identified also includes repetition of the "sh" sound in "sheet" and "shelf," which crams a lot of alliteration in to these two lines:
That white Expanse
reduces to a neat
compression sitting in the smallest space
a sheet can pack in on a cupboard shelf.
It is important to remember the context of what this alliteration is being used to describe. The "Small Town Dance" that is the basis of this poem is the common and everyday activity of two women working together to fold sheets, that would be very difficult if they were trying to do it by themselves. The lines quoted above are used to reflect on the way that something that appears to be so vast and have such a massive "expanse," actually can be folded down to a "neat / compression" that is folded tightly and packed away. The alliteration is used to emphasise the smallness of the sheet as it is folded away. This of course helps to highlight the theme of how women are marginalised and have restricted opportunities. Although it seems they have infinite possibilities, so often their lives are curtailed and folded away tightly just like the sheets they fold away themselves.