Notice that the block is simply made of two different size Triangle & Square blocks,
joined by two rectangles to make a square. For fun, I’ve called these smaller blocks
“brothers”, hence the block name: Big Brother, Little Brother.
Pieces A6 and B6 are optional -- if you prefer a simpler block, cover the areas A4
and A6 with a single rectangular piece of fabric; repeat for areas B4 and B6.