My boyfriends mom and grandma gave soda to our month old baby

As ridiculous as it sounds Yes my boyfriends mom and grandma gave my baby soda which is a big No No I was extremely pissed off and asked them several times not to be giving her soda but they go behind my back and do it anyway I don't live with them anymore me and my boyfriend moved out because of that but his mom still wants to see her grandbaby and my boyfriend dosnt want me pushing them away we've been together for 2years and I really don't want to go back over there I don't know what to do my boyfriend dosnt seem to understand how drangerous that is I'm trying to protect my baby but it's hard when his side of the family always want to see her.