Prayers going out for both of you ... niece and yourself.
I was in denial so bad when my kid was first using meth - she used from 7th grade until 11th grade before I 'knew'. I swore I would never be tricked again. WRONG!!!
This last time, she was using actively, my husband knew it, and I still denied it. He was right, of course.
But the difference was, THIS time I recognized the denial as my safe haven. My denial did not change any outcome and made ME feel better. So I don't really care that I was wrong... at least I wasn't frantic with worry.
Of course, we continued to protect our valuables and bank information.... No need to be TOO trusting.
I hope your niece can find her way into sobriety, and you can renew your relationship with the girl hidden inside the addiction.
((hugs))