To be honest, while I applaud you wanting your niece to be sexually informed, unfortunately at the end of the day, her mother may not want her to be.
And since the child is only 9 years old, her mother's wishes generally overrule most other people's, except in extra ordinary circumstances.
Therefore, I'd advise you to talk to your sister about it first, before you give her the book. For the sake of your relationship with your sister it may be the best thing. If you gave the kid the book without consultation she may just take the book of the child and then blow up at you over it.
At least if you talk it over with her you might be able to convince her about it.
I can't really advise on books suitable for a 9 year old for sex ed. I know I read one called Let's Talk About Sex when I was about 11 or so. I'll try and find it online for you.
Edited to add link:
here's the book:
[This message has been edited by Slayer_gurl (edited 09-23-2005).]