Is it permissible to buy cigarettes or any forbidden thing for one's father if he asks for it? And is it permissible to work in a bank if no other job is available? And is it permissible to divorce a woman and marry her sister during her lifetime?

It is forbidden to buy cigarettes for one's father, as it falls under aiding in sin, unless refraining from doing so would lead to a greater harm; in which case, it is permissible to buy them for him while advising him. However, if he requests something more serious, such as alcohol or drugs, then one must refuse, even if it leads to being expelled from the house, unless the person is unable to support himself due to young age or disability. As for marrying the sister of a divorced wife, it is permissible after her waiting period ('iddah) has ended, because the prohibition of marrying a wife's sister is temporary and ceases when the reason for it is removed.