Two items …
First:
Most rental car companies have pretty much the same features on rental these days though the prices vary all over the place. Unlimited mileage is pretty standard nowadays. Sometimes ultra cheapo deals have mileage limits, but even those are plenty for even extensive in-town use. Not good for multi-hour road trips though.
Bordering state limitations are typical but again the details vary. The last car I rented was from a name brand company in a metro area that straddles a state border. The immediately adjacent state across town was OK, but none of the other states bordering the renting state were allowed. The fact it was a 4 hour drive just to get to any of those other borders was probably a factor.
Definitely ask for all these details.
The second item:
If you are renting the car with the intent of your niece driving it that’s a BIG deal if you don’t tell the rental agency. As in no insurance, car’s technically stolen while she drives it, etc. The rental company absolutely needs to know absolutely everyone who intends to be a driver. And they need to get the driver’s DL info. Usually in person. Expect to pay extra (a lot extra) if she’s not your spouse. Which your niece obviously isn’t. Then expect to pay double extra on that if she’s under age 26 or so.
It pays to ask about all these things.