You should start using that time to instead show them the quality of life they have can improve by not smoking. Lead by example, take up more sports and ask them to participate with you. Find activities that require them to breathe and take part in those activities. If they are shown that the things they want to do with you require them to be in better health, they will do it. If not, I best think to let the idea go really and allow them their vices and as you grow they will allow you your vices and still love you fully anyways.