How do I hold to a $20 family gift limit when someone keeps spending much more?

My sister handed my daughter an $85 toy and said, "I know we said twenty, but I saw it and thought of her."

The adults had agreed to a $20-per-child limit for the family gift exchange. It was my suggestion because buying for all six children had started to make December difficult. Everyone agreed in the chat. I spent around $18 on each of her two children's presents and felt fine about them until the big box appeared.

My daughter was delighted. My sister didn't ask for anything more expensive in return, and I don't think she meant to embarrass me. Still, this is the second year she's gone far over the amount. I now catch myself looking for pricier gifts so my contribution doesn't seem small beside hers.

Would you remind her privately about the cap, change the exchange so adults buy only for their own children, or keep to $20 and work on my reaction? I don't want the children pulled into a dispute over generosity, but I did hope the agreement would remove this comparison.