I wanted it to be a surprise, but my sister let the cat out of the bag.

我想给大家来个惊奇，可我妹妹却先说漏了嘴。

《牛津词典》

Sometimes a market trader would put a cat in his bag and pretend it was a pig to fool the customer! So if someone lets the cat out of the bag, the secret would be open to us.

有时一个市场商人会把一只猫放在他的包里，假装它是一只猪来愚弄顾客！所以如果有人让猫从包里出来，秘密就会为我们所知。

youdao

It's a secret so please don't let the cat out of the bag.

这是一个秘密，因此请不要泄露秘密。

youdao

Just be careful not to let the cat out of the bag, or someone may leave you holding the bag.

VOA: special.2009.01.25

If you tell about something that was supposed to be a secret, we say you let the cat out of the bag.

VOA: special.2009.09.27

The phrase to be left holding the bag is as widely used as the expression to let the cat out of the bag.

VOA: special.2009.01.25

Construction and Housing Minister Ariel Attias let the cat out of the bag.

CENTERFORSECURITYPOLICY: Netanyahu's perilous statecraft

Letting the cat out of the bag a few days early, the regional carrier is re-branding itself as C Spire.

ENGADGET: Cellular South renamed C Spire Wireless, becomes Puff Daddy of regional carriers

They let the cat out of the bag only because there were all these rumors about JFK and other extracurricular activities.

NPR: 'First Off the Tee': White House Golf Tales