What is the Sharia ruling and the solution for someone who sends money to his father, knowing that his siblings take from it and spend it on disobedience to Allah and consuming hashish?

Filial piety is among the best deeds, and assisting one's parents financially is part of this piety, especially if they are in need. Allah has linked His right to their right: "And your Lord has decreed that you shall not worship except Him, and to parents, good treatment." (17:23) Giving money to his children does not necessarily mean that the father knows or approves of them using it to buy hashish. There is no sin in giving him the money, while advising the children and reminding them of the danger of hashish. The father's needs can be purchased directly instead of giving him money, to avoid this problematic situation.