In this excellent short story by D. H. Lawrence, the situation of Mabel is clearly indicated in the opening paragraphs. However, unlike her three brothers, she has no real hope or chance of finding alternative employment at her level, and looks either to face a future working as a menial servant or becoming dependent upon the hospitality of others. Both of these are fates which we can understand she would object to strongly, given her own independence. However, note how their situation is described in the second paragraph:
The three brothers and the sister sat round the desolate breakfast-table, attempting some sort of desultory consultation. The morning's post had given the final tap to the family fortunes, and all was over. The dreary dining-room itself, with its heavy mahogany furniture, looked as if it were waiting to be done away with.
Mabel is thus a young woman without options and resources. It is clear that her brothers care little about her fate and will offer her nothing. As an unmarried woman who has kept house, she finds herself now cast off in the world, friendless and devoted to the memory of a dead mother that, as we shall soon find out, she feels right that she joins. It is only because of the desperate nature of her situation that she feels forced to engage in such a path of self-destruction.