AITA for telling my niece why her birthday party really had to move?

My twelve-year-old niece, Ruby, was supposed to have her birthday at an art studio. She had chosen a painting workshop and invited eight classmates.

Three days before the party, my sister announced that the studio had “messed up the booking,” so the party would be at home instead. Ruby was furious at the studio and wrote an angry review on its page from her tablet.

I knew the studio had not made a mistake. My sister had invited twelve additional relatives after signing the contract. The room limit was firm, and she refused to hold separate cake at home afterward. She canceled and lost the deposit.

When Ruby complained to me that the studio had ruined everything, I said, “They didn’t cancel. Your mom changed the guest list.”

Ruby deleted the review. Then she confronted her parents in front of the relatives arriving to help decorate.

My sister says I humiliated her and dragged a child into an adult budget and guest-list decision. She had planned to explain after the party, when Ruby was less upset. I think allowing Ruby to publicly blame a business for something it did not do was unfair.

Ruby’s home party went well, but she spent the first hour asking which relatives had taken her friends’ places. That was not what I intended, and it hurt people who had accepted an invitation in good faith.

I could have told my sister to make Ruby remove the review without revealing the whole reason. Instead I corrected the lie in the cleanest sentence I had, then left everyone else to live inside it.