Little Dorrit

When Arthur Clennam returns to England after many years abroad, his curiosity is piqued by the presence in his mother's house of a young seamstress, Amy Dorrit. The cold and forbidding Mrs Clennam isn't usually given to charity, so why has she made an exception in taking on Amy? Is she making reparations for a past wrongdoing? Does it have anything to do with Arthur's father's dying words and the pocket watch with its mysterious inscription, 'do not forget'?