Mil issue help?

wen my m.i.l comes to my house she always says oooh he soon be coming with me to stay soon for the night, but there is no way my son will ever be staying at her house the house is too mad she has a lot of visotors half of which i dont know she has cats hairs everywhere and she babysits two other kids and her other children are horrible to them smacking them and swearing and smoking near them, my son is just 5 months old but because i see her a lot i know she is gonner ask me seriosley one day and i dont know what im going to say, i mean she is great with him and i let her babysit only if she comes to my house, but i neva want him staying there but obviosley i want my mum to have him for the night and even for a weekend once hes out of the baby stage so shes gonner be like why can she but not me have him i dunno wat to do help! lol