If I need some words to express my real feeling when you were born then I might use bliss, elation, ecstasy or rapture. I know those are not enough. Wishing happy birthday to lovely niece.

You Are Such An Important Part Of My Life
Happy Birthday To My Precious Sister In Law
I Hope You Have An Exciting Joyous Birthday
Take All The Lovely Greetings On This Special Day
I Wish I Had Been As Pretty As You
You Are My Niece Which Gives You the Official Right
I Wish God Bless You With Success And Long Life With Me
Thanks For hearing The Words I Never Said – Happy Birthday Friend