Partner noticing I don't want MIL babysitting our child

My partner and his mum have a close relationship. They chat on Snapchat (which I find a bit odd but not overly) every day and there are things I don't tell him as he has history of running to his mum behind my back. So anyway the point of this is that when my baby was 7 months old, my MIL met us for a day out and she took my niece and nephew. My niece asked if my baby could have ice cream and I said no. So everyone knew the boundary here. I had gone over to the coffee truck to get us coffee and looked over to where they were sat and my niece was giving my baby ice cream. MIL saw me looking and obviously told her to stop. I didn't say anything cause I hate confrontation but my nephew then told on them anyway and told me that niece was giving baby ice cream, niece then piped up saying 'but granny said it was ok' I said well he isn't allowed ice cream and nothing else was said. It was so awkward, MIL didn't apologise or anything. I've caught her nosying at our mail one time too as she thought I was getting baby strapped in the car and she was at the 'toilet'. I don't trust her fully, I think she can be sneaky and won't respect my boundaries. The problem is now is that my partner is noticing that I allow my own mum to babysit but not her and I feel like if I tell him the reasons, he would tell her even if I ask him not to and this would create a horrible atmosphere. I know most people will say have a conversation and say how you feel but I honestly don't think it would make a difference as she would say what I want to hear but go behind my back anyway.