What is the ruling on someone who swore, saying: "It is forbidden by three [oaths], and she is as forbidden to me as my mother and sister if I smoke," and then he smoked thereafter, while he did not intend divorce or anything else with this oath?

If you do not smoke yourself, nothing is binding upon you. What was uttered by your mother does not necessitate an oath's violation on your part. Prohibiting a foreign woman is idle talk and does not necessitate anything unless one intends ẓihār (a pronouncement of divorce-like prohibition) from her, in which case it becomes binding and the expiation for ẓihār is required if he marries her. If the Islamic marriage contract has already taken place, then she is a wife, and smoking must be quit immediately due to its harms and dangers.