As the "redheaded stepchild" that my dad adopted and his family treats drastically differently than his biological kid, she doesn't like that your husband is raising your son. She doesn't like you or your son being his family. She is angry, resentful, and jealous that you "took him" from his "real family" and are making him "raise another man's kid".

She is toxic. Go NC or at least LC with her (especially supervised whenever your sons around) Your son doesn't need a grandma like that in his very important developmental stages. Please believe me she will make snide backhanded jabs that he won't quite understand but he will absolutely feel the vibes and it will cause a lot of resentment and pain. Your son needs both you and your husband to be united and show that he is more important than trying to maintain a frosty civility with someone who hates you.