Was I out of order with how I handled this?

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I have 3 daughters at home, 1 in her 20s and two teenagers.
The other day, my eldest daughter told the older teen something private. It wasn't anything major, but she wanted to share an experience she'd had and how it made her feel. It wasn't appropriate for the younger teen to know, nor was she supposed to find out. My eldest would have told her if she wanted her to know.
Older teen told the younger one about it. "I tell her everything", she told me. It really wasn't her place to do that. And it was sort of out of titillation.
Well, I told my eldest that she'd told her younger sister. She had also been sussing me out, to see if I knew, but I did as eldest had already confided in me.
I feel my eldest needed to know, so that in future if she chooses to share something with her, she can make the expectation very clear that it's to stay between them. She thought she already had, but her sister claims otherwise. She's furious with her sister (eldest is ASD, so feels strongly about the right and wrong of such things), and middle daughter is raging with me, saying that I can't be trusted to keep a secret. The irony!! Middle has form for not taking responsibility for things.
Was it so wrong of me to tell my 20 something daughter that her younger sister had broken her confidence?
Now nobody is talking and it's awful And I do somehow feel like it's my fault.
AIBU?

YABU. I would've kept well out of that.
They're sisters. Never get involved in their drama.
They're all far to old for you to be controlling their business.

You weren’t wrong and your middle child needs a wake up call. She broke her sister’s confidence and it sounds like she was stirring, though you as her mum will know best on that. Frankly I’d sit her down and give her a good talking too about keeping other people’s news private and hypocracy.

I never control their business
I just thought the whole thing could be a bit of a life lesson for middle child.

Honesty the best policy imo. You were stuck in the middle so something had to give.

Thanks for your thoughts everyone. And I suspect it's not the first time this has happened.

Oldest daughter needs to understand not to ask one sister to keep secret from the other

Absolutely you did the right thing in telling your eldest that the middle one had broken her confidence. It allows your eldest to make decisions as to whether to tell the middle one anything in confidence in future.

Middle child needs to be told that when you're told something in confidence, that it means you stay schtum (unless it's dangerous/safe guarding issues) and you don't blab your mouth off.

This.
She needs telling.
Should she similarly break the confidence of a friend she can expect and will deserve a shit storm.

I agree. You did the right thing. Middle child needs to learn that sometimes some secrets are not yours to share.

Middle child has done wrong, however this is not something I'd have involved myself in.