All of these questions should be answered by AT&T and the answers should be specific to her phone and her plan or the special set up for European travel. There should be a part of the company that deals with overseas travel exclusively. Those are the people to talk to, not just the rep who answers the general number or works in the phone store.
At least that's the way it works with Verizon. The global people have always gotten it right on my trips. The initial reps who answer the main number have always screwed it up. Verizon is typically more expensive than AT&T, but our price is set by them because our phones are global ready and there is no SIM card manipulation.
For Italy this past fall, sending a text from Italy to the US or anywhere else cost us $.50 and receiving one from the US or anywhere else cost $.05. The cost of phone calls was $.99 vs. the standard rate of $1.29 per minute. That was with an additional $10 per phone per month to cover both calling/texting from the US to Italy and from Italy to the US or other European countries. We did not get data because it was too expensive. We just used Wi-Fi. Our phone numbers stayed the same.
Your sister needs to check with AT&T and be persistent until she gets to the right people to talk to about traveling to Italy with her own phone and phone number. And here's another tip: she needs to turn the thing off at night so no one wakes her up in the middle of the night. That's a good time for charging, using a plug adapter.