Recently I read this article, which describes the flexible use of the word cousin in African-American and Latinx communities. It made me wonder: historically, have Jewish communities have had similarly flexible family taxonomies?
Converting from comment to answer:
Genesis 11:29 Mainstream medieval French Jewish commentator, Rashi, holds that Sarah (Abraham's wife) is the same as his niece, Iscah.
19:2 Abraham tells Abimelech that Sarah is his sister. (In a a similar incident,in Egypt, Abraham tells Sarah to say this, implying that it is a fabrication, or at least misleading)
20:12 Abraham justifies what he says as being technically true "And also, indeed, she is my sister, the daughter of my father, but not the daughter of my mother, and she became my wife."
...Now if you ask: Was she not the daughter of his brother? [The answer is that] grandchildren are considered like children (Tosefta, Yev. 8:8; Talmud Bavli, Yev. 62b); therefore, she was (considered as) Terah’s daughter.
And so did he say to Lot, “For we are kinsmen” (אֲנָשִׁים אַחִים) [lit. men, brothers] (although, in fact, Lot was his brother Haran’s son). - [from Pirkei d’Rabbi Eliezer, ch. 36]
Abraham is taking advantage of the loose meaning of the words "sister and brother" to avoid admitting to the King that he lied. At this point, Abraham has no fear of reprisal because the King only found out that something was amiss because God threatened him. Abraham has a big, bad, deity on his side. He could simply tell the king that he lied because the Philistines are a bunch of savages who would try to kill him to take his wife and then flip the King the bird and move his tent somewhere else.
In short: A universally regarded Jewish commentator from 1100 asserts, using older sources (approximately 1000 years older), that Abraham used the words brother and sister in broader terms. These older sources refer to even older traditions.
Hebrew in general, and biblical Hebrew especially are very metaphorical languages.
A well-known phrase is "כל ישראל אחים" meaning "All Israel are brothers". Essentially, the word for brother is easily and commonly used metaphorically to mean kinsman, or even more broadly. Two people in the same shelter can be said to be "brothers in their trouble". Some of these phrases, like "brothers in arms" have passed through into English as well.
As an aside, in modern Hebrew, Arab people are occasionally referred to as "cousins", because the tradition is that they are children of Ishmael, son of Isaac, whereas Jews are children of Jacob, Ishmael's younger twin. This makes the two peoples cousins.