Nonsense Mil

In that case, supervise his interactions with grandma when he is impressionable and young. A third person cannot control much on 2 people talking/interacting especially as your son grows older. By that time hopefully he will learn that bad mouthing others reflects more on the person doing it than the subject of the talk.

 

Tell mil you will teach son mother tongue.
If she wants to teach start with moral stories, Temple's history, annamacharya , shankaracharhya, tenali Rama type story. No family things, who has better money, who is family etc etc.
Kid is young will believe what elder says esp the grandparents make much impression.
My grandparents taught music and devotion. We adhere it till date

 

You use the word nonsense too often.... You can add a few other words to your list of vocabulary

 

Nonsense MIL, nonsense comment. ... Actually for you the world is nonsense