Wait should not it be his brother in law's brother, or (English isn't my first Language) y...

I think you’re focusing on the wrong thing here

Both can be correct, but calling someone your "brother in law's brother" is a more complicated version of saying that you're not very close. It's still his brother in law since he's the brother of the sisters husband :) (I think, I haven't actually read this and just looked at a few pages to see what the family dynamic was haha)

I guess your sister bil can be your bil too