I'm going to tell my niece the truth about the "tooth fairy"
Why would you want to tell your niece the truth about the tooth fairy? Wouldn't you want her to find it out naturally on her own time?
I'm sure she wants the truth.
Why would you of all people be the one to tell your niece about the truth of the tooth fairy? Her parents would become upset with you, and she might not be able to handle this truth just yet. You need to let things happen on their own time and not rush it.
I just want her to be informed.
It is good to want to keep your sister informed and protect her. You should think about why you want to tell her and whether or not it will help or hurt her.