Your mum is being unreasonable, she is an adult and should know not to smoke in front of kids. But laying down the law is never easy with a parent. My suggestion is the next time you see your mum to sit down and have an open conversation about your rules with regards to smoking and your children. Do it face to face so there can be no misunderstanding. It really sucks though. My mum was similar, she thinks holding a cigarette at arms length is ok with kids, and when I asked her not to smoke in her house when we were staying she thought I meant only when the kids were awake so when we went into the living room in the morning the room stunk of smoke. Sadly my son was suffering from lung issues and became very ill after a visit to her house and I was furious and made sure she knew exactly how ill she had made him. We never even went into her house again and met at a mutual location or I stayed with my sister. If you can avoid a situation like that you can save a lot of heartbreak. If your mum won't comply then perhaps you won't be able to visit her in her home any more.