It is Forbidden to Help Another in Sin

Question :

My father smokes, and he often asks me to go out to buy cigarettes for him. Should I obey him? If I do obey him, am I sinning, keeping in mind that if I don't obey him, problems will ensue?

Answer:

It is obligatory for your father to stop smoking because of A the manifold harms that are associated with smoking, and because it is from the Khaba'ith that Allah Almighty mentioned regarding his Prophet ﷺ:
"He allows them as lawful At-Tayibaat (all good and lawful things), and prohibits them as unlawful Al-Khaba'ith (all evil things)."

And Allâh Almighty has permitted only the Tayibaat for His servants, as mentioned in this verse of Surat Al-A'raf.

And his saying in Surat Al-Maidah:
"They ask you what is lawful for them? Say: Lawful unto you are At-Tayibaat (all that is lawful and good)."

Thus there are Al-Khaba'ith and At-Tayibaat, and smoking is from the former kind that is harmful. Your father, and others like him, must repent to Allah Almighty and forsake sitting with those who continue smoking. You are not allowed to help him sin in this situation or in any other that involves sinning. Allâh Almighty said:
"Help you one another in Al-Birr and At-Taqwa (virtue, righteousness, and piety); but do not help one another in sin and transgression. And fear Allah. Verily, Allah is severe in punishment."

You and your brothers and uncles should there be such should advise him and warn him with the previously mentioned verse and the saying of the Prophet ﷺ:
"The religion is sincere advice"

They asked: "O Messenger of Allah! To whom?" He ﷺ said:
"To Allah, His Book, His Messenger, the Imams of the Muslims and their masses."

I ask Allâh to bring your father to what is good and to help him to repent from this and all other acts of disobedience, and to make you one of his helpers in good. Verily, Allah hears all and is near.