Answer:
This excerpt taken from Swift´s famous satire, A modest proposal, elaborates one key feature of his essay, which is the injustices to which the Irish were subjected at the time he wrote it (1729).
Following the excerpt´s context, one can easily conclude that the word sustenance refers to the means of support and subsistence (like food and shelter) that Irish mothers seek for their offspring in the face of unjust labor and social conditions.