Can I I tell my sister her daughter is going to become a stripper?
Isn't that none of your business?
I'm a grown up and I can make a decision for myself.
Not to be rude, but I think you should butt out of this. Ultimately this is your niece's decision, not yours.
I can't let this happen, I refuse to let my niece become a slut.
As long as your niece is an adult she is going to be free to make her own decisions so you may want to not be involved between her and her mom.