As you said, your mum smoked through all her pregnancies (including yours) and you survived, so it’s not the end of the world if your kid spends a weekend at a smoker’s home. But yes, smoking is unhealthy, second hand smoking is unhealthy, and the effects on an infant are greater than those on an adult. This means that if your kid lives with them for 20 years and your mum smokes like a chimney, there’s a higher risk of cancer. But I wouldn’t worry about the odd weekend, especially if your mother just smokes occasionally (out of the mere fear of your reaction). The damage of not seeing your grandparents regularly is probably far greater to a child than your grandmother’s cigarettes.