What is the ruling of the Sharia regarding a girl's question about the extent of the obligation to spend on her siblings and to give her parents a portion of her salary, given her dire need for every penny, in light of her parents' negligence in spending and their drowning in debt?

You are not obligated to spend on your parents or siblings, unless the parents are in need of financial support, or if the parents are unable to provide for your siblings, in which case their financial support becomes your responsibility, as is customary, if you are able. Spending on them is among the best righteous deeds.