Has your sister and niece paid any attention to the news out of airports over the last few days? Delays and cancellations everywhere! Maybe next week will be better, but it’s WINTER. If everything runs smoothly and on time, an hour MIGHT be okay. But the least glitch on her first flight out of Chicago will likely make the connection impossible.
I have flown out of Philadelphia a few times and, if I remember correctly, you can walk between the terminals, but there is quite a distance to cover. Keep in mind that, if your niece is not sitting near the front of the plane, it might take some time to even get off her plane. Also, international flights start boarding a good hour before takeoff, so they may already begin boarding before your niece’s Chicago flight gets to its gate. If your niece arrives at the Rome flight’s gate after they have closed the door, they won’t let her on, even if the plane is still sitting there.
So, I’m with you - a longer layover would be better, but it might be too late (or too expensive) to make a change. At the very least, your niece should plan what she would do if she misses the connection. Yes, the airline has the responsibility to get her on the next available flight - but that flight might be the next day (if there are any seats available on it!) Would she go to an airport hotel? Have friends who can host her? …???