Here is a proverb that is fairly common, but not really among the best known or most intuitive. Dire a nuora perche' suocera intenda. To tell the daughter-in-law so that the mother-in-law finds out. In other words, when you want to get something out (especially something delicate, critical, or even offensive) without confronting the person directly, you tell a third person who is sure to let it out.This obviously relies on the not rare negative perceptions of mothers-in-law, which seem to be cross-cultural.
In the photo, we see a label for the Milk-of-Mother-in-Law liqueur: "one sip and you fall off a cliff," and "previously in use by Malay pirates." The Italians also have pasta and cookies called lingue di suocera (mother-in-law tongues) that are long and rough, with sharp edges.
