My sister is flying American from Philly. She can't walk or stand much. Is there a handicapped line for customs? We know that American is good on service like wheelchairs in the US. Will she be able to get the help she needs in Rome after she lands?

American will arrange for a wheel chair to meet her on lading and they will wheel her through Immigration and luggage retrieval/customs.

Don't wait until she lands in Rome to look for help. You can arrange it ahead of time on the AA website or at her departure airport when she checks in.

I don't know how things are done in the US; but any place else I travelled (by air) with my wheelchair-bound friend, mobility/assistance services had been provided by the respective airport or a company contracted by the airport. So the "quality of assistance" is no different if you fly airline A or B.

You need to make an SSR (Special Service Request) when you book your flights.
Or, when you already made your booking, edit your booking (by internet or phone) to attach the respective SSR to your existing booking.

SSRs come in standardized IATA categories worldwide, so the level of assistance is (should be) always the same.

IMO, your sister should ask for WCHR category assistance which means she can walk steps (only important when plane should have an apron position and you literally need to climb down or up a gangway), can walk onto and inside the plane, but needs assistance to walk distances within the airport.

To be on the safe side, it does not hurt to address cabin staff after boarding the flight to Rome to double check that the SSR/WCHR status has made it on that flight's passenger list.