Hello and welcome to Apple Support Communities, tazbo4.
If we understand your post correctly, on your iPhone it appears as though your niece is the family organizer but it should be you. We’d like to help.
Let's start by checking which Apple ID is signed in on your iPhone. On your iPhone, tap Settings. At the top of the screen you should see your own name. When you then tap your name you should see your Apple ID email address under your name.
If, instead, it's your niece's name and Apple ID you'll need to scroll to the bottom of the screen and have her sign out and then sign in with your own Apple ID. Keep in mind that if her Apple ID was signed in on your iPhone your contacts, calendars, and notes may be saved to her Apple ID.
If your name is already there as expected (or after you've signed her out and signed in with your own Apple ID) then we can move on to the Family Share issue.
If your niece is set as the organizer and it needs to be you the current Family Share Group will need to be disbanded and then a new Family Share Group started from your Apple ID.
To disband the Family Share Group follow the steps in this article: Leave Family Sharing
Once that's done you can set up a Family Sharing Group with yourself as the organizer with these steps:
Set up Family Sharing
Let us know if that resolves this issue for you.
Thanks for using the Apple Support Communities.