Sorry to upset you. Some people have agreed and some have disagreed, so it's up to you how you take it or see truth in it.
But yes, really - I kept expecting you to say "the older two are from his first marriage", which would have explained everything about the way you wrote about them, like they were nothing to do with you and were really your DH's problem as he spoiled them.
Look; they took the younger children's chocolate, it was cruel and stupid and teenagery of them. But take it down a notch on the drama, they didn't nail a puppy to the wall or burn the house down with the little ones locked inside, they went too far in a vindictive joke.
I'd have been furious too but I'd have asked myself why they did that. Why did they want to hurt their siblings? Why is it an "us" and "them" situation? Why the attention-seeking? An age gap alone doesn't explain that, it has to be something else.
I disagree with others on something else too: your MIL sounds overly strict and dramatically over-involved to me.
You didn't need the entire family to pile on and and tell them off and remove lifelong benefits like learning to drive - how does that fix anything? No wonder your husband reverted and sympathised with them again. Your DH is probably still rebelling against his mum and you by extension. Not always fun to live with I imagine, but you're part of all this dynamic too.