Is it permissible to demand compensation from the person who caused my brother's death to pay off his study debt? And is it permissible to use insurance money as an ongoing charity for him and to help my sister get married?

If someone accidentally hits and causes the death of another, two things are required of them: the blood money (Diyyah), which is one hundred camels or their equivalent of one hundred thousand Riyals. This is borne by the 'Aqilah (male relatives on the father's side), and if that is not possible, then it falls upon the killer. The second is expiation (Kaffarah), which is fasting for two consecutive months.

The guardians of the deceased may waive the blood money if they are adult and of sound mind. However, if the deceased had debts, they are not permitted to waive it; rather, they must take the blood money first to settle his debts, then distribute the remainder among his heirs.

If an insurance company pays the blood money or compensation, you are permitted to take it. After settling the deceased's debt, you may donate the remainder as ongoing charity (Sadaqah Jariyah) on his behalf or use it to help marry off your sister, as the heirs have the right to dispose of their shares after the debt has been settled.