What does religion impose on a brother regarding his unmarried sister who lives alone after the death of their parents, knowing that earning a livelihood is only possible in tobacco factories in her country?

Your duty towards your sister is to maintain ties with her and not sever them, and to spend on her if she is in need and you are able. As for her living alone, if there is a fear of fitna (temptation/strife) for her, it is not permissible to leave her, and you must house her with one of her mahrams (unmarriageable kin) or a righteous woman. If fitna is not feared, then it is permissible, but it is better that she does not spend the night alone. You should also search for a righteous husband for her, and there is no harm in proposing her to a man for marriage. With a reminder that working in tobacco factories is not permissible.