Dear mummy, it doesn’t sound like you did anything wrong. If me, I will just give her either water or just pack drinks.. If she complain, then she won’t get any drinks from me anymore and I will just tell her off “you are just creating unnecessary problem at my house, I don’t see why I should still serve you drinks but not to worry that your son will bring it to you”.
This type of MIL needs to teach them a lesson and don’t let her climb over your head! Especially when you have kids I am very sure she will make sure she involves every single things and wants you to be her follower.
Be strong mummy! 😊