Mother Issue

I'll be visiting my parents next month, with my baby for the first time. They're very excited to finally meet our baby & we have gone out of our way to make it happen (as we live in a different state & they're unable to travel). My mother in the past has fed my niece & nephews really unhealthy stuff. Like for example, she gave coffee to one of my infant nieces & when I told her it wasn't okay she said "isn't no big deal" (she would also give them soda etc.) My problem with my mother is that although I have strictly told her that I do not want my baby (which is only four months old) to consume that junk (I only want her to consume my breastmilk), she'll make comments about how she'll do it when I'm not watching. What should I do? I really want them to meet my baby but I'm conserned that she'll actually do it. Any advice?