How Crazy Are the Crowds in Rome?

The key to these popular sights is time of day. Arrive, early, very early to avoid the bulk of the crowds.

No where is quite as bad as the Sistine Chapel. It is such a confined space.
The other 2 places I would have some worry for you would be at the Trevi Fountain and at the Spanish Steps.
You can see both walking from afar and avoid the main crowd but those 2 sites can get really crowded, seeing those early morning is a good idea for you, they are free open air sites.

Other than at the initial entrance the Colloseum is not bad and crowds are much lighter at the Forum and will be as well at the 2 museums. These places are all much more spread out to better handle crowds.
Did you go to St. Peter's ? Shame if you did not try it, I would say crowds their are worse than the Collosuem but still nothing like Sistine Chapel.

We've been twice to the Colosseum, and though you may find some crowding around displays or if you are in a group, I found that people are pretty spread out - ditto the Forum - it's all outside, so lots of room to spread. And we did the Vatican museum in 2014 and took an after hours tour and it was really crowded, so comparatively, she SHOULD be fine at Col and Forum (not sure the other museums).

If you still want to see the Sistine Chapel and Vatican museums, I suggest you book an early access tour such as this one with Dark Rome:

https://darkrome.com/vatican-tours/sistine-chapel-early-access

They still have space for tomorrow, andprobablynSaturday as well.

We did this when in Rome a year ago and practically had the whole place to ourselves---there were fewer than 50 people in the chapel, and none in the galleries we walked through, apart from the Rafael Rooms. We left around 10 am and people were streaming in.

I could not enter the Pantheon when we were there because of the crush of people. But I had no trouble finding space in the Forum.

The Forums and Colosseum are easier to handle because they are relatively open spaces - even in the Colosseum, when you are out of the walkways, you have sky above you and can usually find a place to move away from crowds.

For the Forums, you can see how crowded it is from the street level. If you decide to go in, buy your ticket and enter at the Palatine entrance - shorter lines.

I'm thinking they're aren't many major tourists sites with few tourists - that's why they're "major" because they draw so many tourists. Maybe you all should be out and about in the countryside with fewer major tourist sites and more open countryside. Think Swiss Alps and alpine villages.

Thanks everyone for the extremely helpful responses.

Has anybody been to Palazzo Farnese? What did you think about it? Some say it's great, up close & personal Renaissance architecture, and others say meh. It's a stone's throw from the apartment we'll be staying at near Campo de' Fiori in February, so I think we may visit. And they do an English tour only on Wednesdays at 17:00.

Jay - We have toured Palazzo Farnese and found it fascinating. You must reserve several weeks in advance.

We were there a week and a half ago and the Vatican and Sistene chappel were quite busy. Maybe they issue a lot of tickets for each time. Around that time we did the Colliseum and Forum. We had walked to the Colliseum first and the lines were very long, so we went to the Forum instead. It was not crowded at all, and then it was late afternoon by the time we got to the Colliseum, and by then the line wasn't very long there either, and it wasn't very crowded inside. I think your late afternoon is generally a good strategy, and nowhere are the crowds as bad as the sistene chapel.

Note, the Sistene Chapel was much worse than the rest of the Vatican tour. It seemed like there were a lot of people just sitting and standing around doing nothing. They weren't even looking at the art, just talking to each other or waiting for other people. Perhaps they were waiting for the rest of their tour groups to arrive? I think they could do a much better job of managing the crowds in there.

Another thing to throw into the hat is to seek out less visited spaces. We drop into churches randomly and have seen the most amazing art by da Vinci and others that still sits in the churches it was commissioned for. Sometimes they are free, sometimes a small fee, but always worth a visit and not crowded.
Do whatever you can to reserve tickets ahead of time. Her anxiety might be reduced a bit if the wait time in a crowded queue isn't so long.
And focus on those outdoor sites! Trajan's column, the Forum, constantine's arch are all out in the open and ready to view.

One of the more pleasant venues in the area is Ostia Antica, the excavation of ancient Rome's seaport town. It's a large area with surprisingly few visitors. Fragrant umbrella pine trees provide welcome shade over much of the site. When we arrived at mid-day on a Friday in May last year, there were two (count them) people in line at the ticket booth. Access is easy and inexpensive via train (Lido line from Piramide station), part of Rome's metropolitan transit system.

IMHO, it's 80% of a Pompeii-like experience for 20% of the hassle.

MACRO and MAXXI are not on most people's list. Are you really into modern art?

Some less crowded museums and sites are:
Capitolini
Palazzo Barberini
Palazzo Massimo/National Museum of Rome
Centrale Montemartini
Domus Aurea (must book well in advance, only Sat-Sun)
San Paolo Fuori Le Mura
Terme di Caracalla
Terme Diocleziano
Santa Maria Degli Angeli e Martiri

Really, there's no shortage of cool stuff to see and do in Rome that doesn't involve masses of humanity. Rick's book on Rome has excellent recommendations.

I wonder if the Sistine Chapel will ultimately go the way of Da Vinci's Last Supper, where reservations are limited and made months in advance, etc. I'm sure those that do make the efforts to reserve for Last Supper, probably to get their time/energy/money's worth.

Similar to what others have said, there are many, many lesser known sites all around Rome that are far less crowded but also offer a great experience, perhaps without the fame. Coming from the States, just about any cathedral in a place like Rome is an amazing experience.