grandparents being disrespectful

Anyone dealing with disrespectful grandparents? 
My MIL has flat out ignored me heaps of times and I know my baby has no idea at this point but I feel if it continues my baby will learn to go against my wishes when he is older. 
For example I asked not to give my baby (6 months old at the time) chocolate dessert but got told 'it won't hurt' and fed it to him anyway. 
I'm worried in future if my son hears me say no and then he gets it anyway he will learn he can still get what he wants. 
Advice please!

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