What benefit is there to telling everyone this forged document isn’t a forged document?
The kanji and the word are exactly the same. That means they share meanings as well.
Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you:
It turns out she is a fake. Your niece (にせ) was a robot this whole time, just pretending to be your niece. What is wrong with this world, where your niece can be an imitation niece. A 偽niece, if you will.