to think MIL shouldn't teach DS that throwing a tantrum is how to get what he wants?

[TickTockGoesTheClock]

A few things to set the context...

DS is three. I've been quite strict in what he eats - he's only had chocolate a handful of times (and the majority of those times were through nursery slip-ups) and until this year, we hadn't bought him an Easter egg. This is the first time I have actually bought and gave him chocolate.

Right, so this afternoon we went to the in-laws. Everyone knows DS isn't allowed chocolate/junk/etc. When we arrived, MIL and FIL said they'd bought DS an Easter egg - I said we'd already got him one, but thanks anyway, DH can eat it.

Later on, MIL decides to present the Easter egg to DS. I explained to DS that he already had half an Easter egg already today, and he can have the rest of the one we bought him later on when we get home. DS was a bit moany, but wasn't too bothered so I put the egg in my bag. MIL chirps in, asking where the Easter egg is, and said "DS you should say, 'I want my Easter egg!' and stamp your feet. Then Mummy will give it to you" [buhmm] She didn't leave it at that, she started looking around for the Easter egg, continuing to encourage DS to throw a strop and saying "don't listen to Mummy, Grandma said you can have it". She only stopped when her mother gave her a stare and changed the topic to distract DS.

AIBU to think whether or not you agree with my parenting rules, you shouldn't try and show me up in front of DS and encourage him to rebel against me?

She was unreasonable........ but you were downright rude and sound like a joyless pain in the arse.

Having read they thread again, still think yabvu in your rudeness. Yes mil was unreasonable too, but your attitude to her was very and unnecessarily rude.

I posted on the other chocolate thread that PIL bought DS (11months) an egg. He is intolerant to dairy and it makes him quite uncomfortable and ill, but they still asked (as if it were me that was the problem,) if he'd be 'allowed' to have any. I just said thank-you very much, took it and DH ate it later.
I will probably reiterate later that he won't be having milk chocolate, or any dairy, but I will wait until another time.
It sounds as if there's some bad feeling between you and MIL which is unpleasant for both of you, so I'd be tackling that tbh.

YABU to refuse the egg! Let children be children and let them have occasional treats ffs.

The MIL was being unreasonable undermining you like that. But perhaps she was pissed off you refused the egg she had bought.

Your MIL was in the wrong but you were being very rude to her. I think it quite normal for a grandparent to give a her grandchild an egg at easter and you were ungracious to say you would give it to your DH instead.