Is one sinful if he stops helping his sister who is raising two orphans, knowing that one of the orphans is 19 years old, smokes, consumes intoxicants, and does not work, and is he obliged to wait for a request for help or should he take the initiative to offer it?

You are obligated to financially support your poor sister if you are capable of doing so, and it is not permissible to withhold it from her. As for your nephew who is able to earn a living, it is his responsibility to do so. His immorality should not prevent you from helping your sister; rather, you should cooperate with her to guide him, steer him away from evil, and provide him with good company, while emphasizing to your sister that she should not use your money to enable his corruption. It is permissible to give zakat (definition) to a poor transgressor if he does not use it to aid his transgression. However, if it is known that he will spend it on an act of disobedience, then it is not permissible to give it to him. One way to circumvent this is by directly purchasing household necessities for your sister, or by having them appoint you to buy what they need with the zakat money. But if the transgressor does not use zakat for disobedience, then it should be given to him along with advice.