Yes — this matches a well-known crude Persian saying that is common in Iranian culture.
The expression is “خواهر زن نون زیر کبابه” (khāhar-zan nūn-e zīr-e kabāb-e), or in more colloquial form “نون زیر کباب.” It literally means “the wife’s sister is the bread under the kebab.”
The joke/meaning
When you grill kebab, the meat (the “kabob”) sits on or drips onto a piece of bread underneath. The bread soaks up all the fat, juices, and flavor, so a lot of people jokingly (or lecherously) claim the bread ends up tastier than the meat itself.
In the saying:
The wife = the kebab
The wife’s sister = the bread underneath
Implication: the sister-in-law is (supposedly) even more desirable / “sweeter” than your own wife. It’s a classic crude male joke about the forbidden appeal of the wife’s sister.
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