wow! i hope your sister has a biiiiiiiiiiiiiiiiiiig bed. :lol:
fill the blank spaces with bigger diamonds and/or triangles
if you make big diamonds - the size of each of the diamond sets that make up the star, it'll turn into a gihumongous hexagon. you can build around that with triangles, more diamonds ... whatever strikes your fancy ... until it's squarish. then start your borders or companion blocks.
clear as mud?