Fake, Imitation
noun, の adjective
The kanji and the word are exactly the same. That means they share meanings as well.
にせ
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.
どうして俺が偽札を偽札じゃないということが皆の為になるのさ？
What benefit is there to telling everyone this counterfeit money isn’t counterfeit?