Use the adjective flaxen to describe things that are a pale golden-yellow color, like your brother's flaxen hair or flaxen wheat fields glowing in the summer sun.
There's a literary history of using flaxen for a young girl's blonde hair that dates back at least as far as the 1500s. This word literally means "made of flax" or "the color of flax." Flax is a plant used to make a linen-like material also called flax, and flaxen originally described things made of this fabric. Today it's mainly used for hair or horses — the flaxen gene causes chestnut-colored horses to have blond tails.