A niece is an ervoh like any other girl
The term “ervah” is usually used to refer to a potential partner who is forbidden to you by virtue of being already married, a relative, or a relative of a previous or current spouse. In that sense, a niece is NOT an ervah*, nor is “any other girl.”
The Wolf
* Yes, technically, if a Niddah, she is an ervah, but when discussing potentially permitted relationships, we don’t use the term ervah in that sense.