Feeding baby food without parents permission

Has anyone else encountered this yet? My SO mom has fed my baby tastes of coleslaw dressing, oatmeal, etc more than once. This last weekend we were at a family cookout and she had fed her coleslaw dressing and then announced it to everyone "mommy, she likes coleslaw" I then said, "you know typically you ask before feeding someone else's baby." And she said, "well it's a grandma's right". I said, "did you birth her? NO. it isn't your right" it was embarrassing that I had to stand up to her in front of her family that's I've only met a handful of times. (Extended family) she does not listen as this wasn't the first time this has happened. We as her parents wanted to do her first taste of new things. She is going to be our only child so we wanted to be the ones to experience all of her "firsts" or at least be present for them. Suggestions? My SO was supposed to talk to her but that's doubtful.