an uncle marrying a niece
That is a much more extreme case and usually implies that they are of similar age so more like cousins anyway.
I know of a case where this happened. You can look at it in many ways: the choson who was the youngest of a very large family, married his niece (his oldest brother’s daughter) who was his age. So (a) His brother became his father in law!! (Can he still call him by his first name?) and (b) looking at it from his parents viewpoint, their son married their granddaughter making their oldest son into their mechutan, and when pg the young couple have kids, they are both their grandchildren and great grandchildren simultaneously. The choson’s siblings (kallah’s uncles & aunts) become her brothers/sisters in law, while the kallah’s siblings (choson’s nieces/nephews) become his brothers/sisters in law. What a mess!