If bluegrass is defined by the instrumentation, it's not the instruments themselves but the way they're played. That's why banjo and fiddle contests typically have separate categories for bluegrass and old time banjo, the same for fiddle.

Also, bluegrass of the forties (Louvin Brothers, Osborne brothers etc.) is a far different thing than bluegrass of the twenty first century so it's hard to generalize about it.

Finally, close or tight harmony is universally sung in thirds by two (or more) people whose voices are similar in range and timber and who have similar speech patterns. That's why they're often brothers or people who've been married forever.

Bev and Jerry