Your brother will be able to look at his ethnicity on ancestry by parent. It’ll just be ‘Parent 1’ and ‘Parent 2’ and it’ll only show what he’s inherited, not what their results would actually be (For example my mother has 20% Scottish and I have none, but a lot of DNA I’ve got from her is bits they think are Welsh. It’s just random which buts you inherit and not 50/50.

The ethnicity is also constantly updating. Matches are 100% scientific. Ethnicity is an educated guess. They’re not going to mistake a Frenchman as Chinese but they might think he’s from a neighbouring European country as there has historically been so much mixing that it’s actually quite hard to pin it down. They’re just comparing your dna to the dna of people on a reference panel who have good genealogy research showing where they’re from. Which is great until someone in their tree actually had an affair, or they’ve gone back x number of generations as 100% French but if they’d gone one generation further back they’d have found they were from Germany… I like to think of it as a very good clue, but not good enough to have an identity crisis over.