I was just thinking about when mc’s marry sisters or twins. Then I realized that their kids will be brothers/sisters and cousins at the same time.
Take it even further and suppose that two identical twin sisters marry two identical twin brothers. Their children practically become genetic siblings or as I'd like to call them, "giblings".
Now take it a step further, the two sets of twins frequently swap partners and all four live as a family unit. The only sure thing at that point is which woman gave birth to which child.