Be polite and friendly
Before you convey your message to a sibling about the effects of smoking and why he or she should quit it, you need to understand that only a friendly and polite tone is going to help the cause. If you try to be authoritative or harsh, it will actually have a negative and opposite impact.
Give examples of others
In almost every family or social gathering, you can find people who badly affected their life due to smoking. You can start to persuade your sibling(s) by giving examples of such people. It is surely going to have a strong impact on the mind of your sibling.
Prove it medically
You need to do proper research about the effects of smoking on health and communicate that to your sibling. Health is one of the most important things for everyone and nobody wants to spend his/her old age with heart and lung diseases.
Get your sibling indulged in a physical activity
Most smokers do not indulge themselves in any physical activity. Therefore, you should try to make your sibling go to gym or swimming club with you. He or she can even start with walk or light jogging. A physically tiring activity is surely going to go a long way to help your sibling quit smoking.
Being patient while trying to convince your sibling to quit smoking is the key. Do not expect that you will deliver a nice lecture on smoking and your brother or sister will stop smoking immediately. A smoker is likely to take time before getting rid of this habit. Remain patient and be gradual while persuading a sibling against the curse of smoking.