What is the ruling on spending on a sister who requests financial support after leaving her job, knowing that she has little savings and that her maintenance might harm the questioner's wife, whose arrival is awaited?

If your sister has money, you are not obligated to spend on her. Otherwise, you are obligated if she has no money, or if what she possesses runs out, and she cannot find work, because spending is obligatory for every close relative who is an heir. The obligation to spend on her comes after fulfilling your essential needs. Be keen to help her in all circumstances, as it is a form of righteousness and maintaining kinship ties. The best advice you can offer is to strive to get her married.