I bet your niece is having an amazing time. Rome was incredible and very different and there was lots to see and do. There were lots of Americans in my hotel and in the city. Rome (like every city) is ideal for long weekends to me so it's a long way to go if you live in the US. Although I would assume most stay for a few days then move on to somewhere else. One US family I spoke to were staying a few days then moving on to Cyprus for your typical beach holiday.
We were told the Vatican was boring by a few people so didn't visit but from the pics I have seen it looks amazing.
Next year I hope to visit the US. Hopefully somewhere hot for 2 weeks but even a long weekend to New York would be amazing