Answer:
When it comes in, From sea to shore, Twenty paces you'll see, No less, no more.
It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter.
I can be short and sometimes hot. When displayed, I rarely impress.
I scribble forms of the finest letter, And repel elements of the harshest weather. I am an arrow-aimer and a dust-breaker.
White fat and fluffy, May even look like a puppy. Seems heavy but is light, Floats about without a worry, But if it turns dark you have to hurry!
I am your mother's brother's only brother in law. Who am I?