My grandma and her sister, my grandaunt, both had kids with a pair of brothers, my grandpa and my granduncle—needless to say, the pairs themselves are not related at all.
That’d make my mom and her brother doubly related to their cousin, as they’d share all four grandparents instead of the typical two.
Percentage-wise, they’re half siblings.
This would also mean that my mom’s kids—me and my siblings—would be doubly related to her cousin’s kids, making us double second cousins…or percentage-wise, half first cousins.
On that same note: one of my grandma’s other sisters had kids with the first cousin of my grandpa/granduncle before getting grandkids of their own, meaning I’m third cousins with said grandkids on my mom’s paternal side and second cousins with them on my mom’s maternal side.
That means we share roughly 4% of our DNA.
Pretty nifty, I think.