If you are looking for advice, I'd say that very few of the six year olds I have known would get this. When you say "What is it like to live in the middle of nowhere" it is difficult for a six year old to figure out that your question actually has nothing to do with what it is like. I usually find it best to stick with something that can readily be interpreted as its actual meaning. Making the leap from the meaning of the word to the spelling of the word is usually hard enough at that age.
Rhymes work well at that age. They remember them and repeat them which can help with solving the riddle. If you can work your niece's name into the rhyme, she will love it even if she can't solve the riddle.
Keep it simple. Try to make it personal. Off the top of my head:
You'll find me in nothing or something or other.
I'm not in your sister, I am in your brother.
I'm not in your grasp, I am in your hand.
I'm there when you crouch but not when you stand.
I'm heard in your height but not in your weight
Hurry and guess me before it's too late.
Write it out for her so she has a chance to see the h's. Maybe be prepared with something extra in case she's struggling:
Here's a little help
If you're having a hard time
I hope you hear the hint
In this happy helpful rhyme.