Notice that the block is simply made of two different size “X” (also known as Broken
Dishes) blocks, joined by two rectangles to make a square. For fun, I’ve called
these smaller blocks “sisters”, hence the block name: Big Sister, Little Sister.
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.