Answer:
I am a box that holds keys without locks, yet they can unlock your soul.
I'm white, I'm round, but not always around. Sometimes you see me, sometimes you don't.
I scribble forms of the finest letter, And repel elements of the harshest weather. I am an arrow-aimer and a dust-breaker.
I know a word of letters three. Add two, and fewer there will be!
Hold these between your knees if you want to set a Cuban beat.
I am your mother's brother's only brother in law. Who am I?