There is an app that will tell you and another person with the same app how closely related you are and will show the ancestry chain between the two of you. It turns out the my wife and I are 10th cousins (common great-whatever grandparents in the 1600s). I started calling her my cousin-wife, but she didn't think it was funny, so I had to stop ;-)

10th cousin is basically “not related”. That same app (Relative Finder) says that Obama is my 10th cousin.

What is the app called?

Isn't this roughly how people talk about their relationship with Native American tribes? They don't do the log2 math, but they're happy to hit you with the fraction.

Certainly, First Nation People of Australia had a language of their own for this:

https://en.wikipedia.org/wiki/Australian_Aboriginal_kinship

You're probably thinking of "blood quantum." It's... complicated.

https://www.npr.org/sections/codeswitch/2018/02/09/583987261...

I come from an old Jewish family that's supposed to come King David, and I recently met a girl who comes from another family like that (rabbinical sources describe a dozen or so families with reasonable claims to Davidic lineage). We used to joke that we're longest removed siblings ever, and now I'm curious about what that natural logarithm would actually look like.