Help me know what to say when undermined

My MIL undermines us right in front of our faces when it comes to our kids.

Example: at the park with my 18mo and my 9 year old niece a few days ago. My MIL had a diet soda with maraschino cherries. My daughter picked it up and said “want some”. I said “No”. (We have a no sugar before 3 yo rule in our house that is well known by our families and basically our only rule.) Then 9 year old said she wanted some, her mom said no, my MIL, then let her have some. The 9 yo took a sip making eye contact with her mom. Her mom got upset with the kid for making eye contact but I was annoyed with my MIL for ignoring her daughter’s parenting decision.

Apparently my little wanted ice from her cup so my MIL started digging ice chunks. Then my little asked for cherries. We give her cherries all the time but the sugar candied cherries are of course off limits from our no sugar rule. I said no, and my MIL proceeded to give her cherries. I kept saying stuff like “okay that’s enough” “that was the last one” and “okay! No more!” And she ignored me every time giving my baby all of her candied cherries.

That was days ago and I woke up pissed off about it. Mad at myself for not slapping them out of her hand. So 🤬 disrespectful.

So I need to prepare for the future. I’m not in anyway a pushover. I’m the blunt abrasive type and I was trying not to yell at her in front of the kids. But never again. Next time, (because their will be a next time), I need to have a calm but firm response on hand. I’ll definitely start with reiterating the rule “No sugar before 3.” But I want to point out her undermining and disrespectful actions to her without yelling at her.

Side note, my BIL told me years ago to “get used to” her being disrespectful and undermining us so it’s not just me. She does that to all the parents.

P.s. I’m not here to defend our no sugar before 3 house rule. Sometimes these posts can have some unrelated hateful comments. ✌️TIA!!!