I like to think of it as the DNA of their people being closely related in a genetic population sense and that plus shared culture and language makes them as close related for countries and thus "brothers", but it doesn't literally mean they share 50% of each others DNA anymore than the average Icelander and the average Norwegian do.

But man if they literally are then I want to see the receipts on how Russia is Belarus&Ukrain's brother since Russia has so many freaking haplogroup and genetic markers because of all it's unique ethnic groups. He'd be such a genetic Mary-Sue, literally everyone's brother or cousin.