How do I convince a relative to quit smoking while pregnant?

My father and step-mother are expecting their first child in 6 months and I’m really excited about it. HOWEVER, my step-mom has not quit smoking and isn’t showing any signs of planning to quit. It has me really upset because I know how harmful that can be for the baby and I really want my brother or sister to be healthy, My step-mother has already had one miscarriage within the last year and we all want to avoid that happening again, but I don’t know how to bring up how much her smoking is bothering me. Any suggestions?