Have fun, this sounds like a great trip for you and your mom and your sis.
My advice for the plane is to dress in comfortable layers. Sometimes it can get chilly on the plane, so you would want a nice heavy sweater, and I would suggest a cardigan, because it’s easier to get on and off than a pullover. The airline gives you blankets, but I’ve always found them to be a little light. Comfortable shoes that you can easily slip off are helpful, and bring a pair of slippers or clean, heavy socks.
I’m a little old-fashioned in this regard, but I personally am not nuts about wearing sweatpants and the like on the plane. Plenty of people do, and there is nothing really wrong with it, it is certainly comfortable, but (again, personally) I don’t like looking like I’m wearing my pyjamas in public. You will see people wearing sweats on the plane, and in my experience, they are most often Americans. I will however take a stand on icky socks (not to imply that you, personally, have icky socks), I have been amazed at people who take off their shoes on the plane, to reveal their icky, hole-ridden, stinky socks. Hence my advice on the slippers.
For Italy itself, I absolutely agree with capybara that even though laundromats are plentiful, it’s more hassle and time than it’s worth. I suggest carefully planning the clothing you pack, so that you can get the most use from the fewest amount of clothes. Khaki slacks are great for this, also, if you have one or two sweaters in neutral colors. The things I would bring the most of are socks, underwear, and T-shirts, and also a few turtlenecks. If you are a woman, bring a few pairs of heavy tights, they are also good for wearing under your slacks because …
I am always cold in Italy at this time of the year. Many places are not heated as warmly as Americans are generally used to, so having your failsafe sweater is helpful. Bring something warm to sleep in, and again, slippers and heavy socks. If you are visiting a lot of churches or historical sites, like castles, they are usually quite chilly inside, and the stone floors can make it feel like the cold is creeping directly into your feet. I second the suggestion of a waterproof rain jacket, and a travel (compact, small) umbrella. It might be helpful to bring a waterproof tote bag folded up in your luggage, this is also good if you purchase some souvenirs and need an additional bag for the trip home. It is great if this is the kind with the zipper because …
Italy, I have found, is generally a very safe country, but there are unsavory people who target tourists, and as you mentioned, you will be getting on and off a tour bus. Pickpocketting is an art form, you will not notice right away if you are a victim of this crime. I recommend carrying bags that zip securely. Do not carry all your cash in the same place – keep some emergency cash in an inside pocket, that sort of thing. If you carry an over-the-shoulder type of bag, carry it so your bag is slung in front of you, not behind you.
Check the exchange rate offered by your credit card, in many cases, you can get a (slightly) better rate charging things to your credit card, so even if you tend to be a “cash on the barrelhead” person, charging can be to your benefit. Visa is most widely accepted in Italy, although the other major credit cards are fairly easy to use.
It looks like a really nice tour! Here are a few quite thoughts about the cities on the tour:
Venice – gorgeous, but strangely a very tourism-based town. Every shop will have the same things for sale. All seafood in Venice is great, this is the place to take advantage of eating seafood.
Milan – has the best (IMHO) higher-end (like high fashion) shopping. I think Milan is a little pricey, myself.
Florence – this is the city I know the best. One nice thing about Florence is that you can pick up some very nice souvenirs in the open-air markets, they are cheap, cheap, cheap, but the quality of items is generally very good. The markets sell designer knock-offs, which are fun to buy, and the actual quality of said items is much better than what you think of when you imagine an American designer knock-off. The best gelatto is sold at Vivoli (ths is the most important thing to know about Italy), the second best at Gran Ducca. Gran Ducca is more centally located. It is your moral obligation to sample all the gelatto available to you so you can make informed comparisons. 
You mentioned you have a son, and Florence has a lot of neat children’s toys and other kid-popular items. People talk about bartering in the markets, but I have always found this to be dubious advice. The prices are generally reasonable to begin with, and the vendors’ goal is usually to get you in and out quickly. The best way to get a price reduced is to buy in bulk – if you, Mom and sis are all buying handbags, for example, get them at the same place and you can usually get the price lowered for the lot.
Pisa (and I suspect this is especially true of Pisa on a guided tour) is mostly about the Tower, as it should be. They aren’t kidding, that thing is leaning! (It’s more alarming when you see it in person.) There are about a million Pisa gift shops/stands in the area, the stuff is mostly junky and over-priced.
Rome – very fun, modern and ancient at the same time, very fast, cosmopolitan, urban pace. Rome can be overwhelming, I would plan to take care of all of my shopping and things like that before Rome, especially if it is toward the end of your tour, you might be a little tuckered out by the time you get to Rome.
One weird thing – it looks, from the web site you linked to, that you are on your own for lunch. I would suggest that you keep fairly low expectations for lunch. In Italy, the general thought (one that I share) is that a good meal is not worth having unless it’s four courses and two wines and takes at least two hours to eat. “Lunch on the go” is not inherently part of the Italian culture, and most places near major tourist attractions are understandably catering to tourists. You know how you can often eyeball a restaurant and its menu and make an accurate guess if it’s the sort of thing you would like? This ability is cultural (I have developed a whole theory about this), and won’t work for you in Italy (I mean, if you’re not Italian)-- it took me years to be able to pick up the “feel” of a restaurant in Italy, so don’t be too bummed out if you end up picking a not-so-great spot for lunch. Your tour guide people will probably be helpful with this, based on what other people have liked in the past.
Pizza is a good, quick lunch, although very different from American pizza. Likewise, coffee and Italian pastry makes a great lunch. Or more gelatto. Hey, it’s vacation! Sandwiches are also available aplenty, although when people rave about great Italian food they gnerally aren’t talking about the sandwiches. This is not to say that you won’t get great sandwiches or other lunches in Italy, but it’s really the luck of the draw, you will probably also end up with some bland sandwiches. I would suggest light lunches, and save your appetite for dinner. Ham in Italy is an art, if you like ham (and pork in general), this will be great. If you don’t eat pork for religious or other reasons, it’s important to ask very clearly first – they sneak pork into a lot of things that you might not think would contain pork.
What we think of as “green salad” is ensalata verde in Italy – to Italians, this literally means just the salad greens, so generally just lettuce. I know a lot of people who were surprised upon receiving a bowl of lettuce. A garden salad is ensalata mista (“mixed”) and what you probably want. Oh! Get French fries, especially in Florence. Some Americans avoid them because they think “oh, I will look like an American tourist if I order French fries.” However, they are prepared with olive oil, very light, and if you are a French fry fan, absolutely great and different from American fries. This is a good side dish to order for the table. A lot of places also offer platters with fried potatoes, zucchini, and any other veggies they have around – this is very good. Many restaurants will serve just the entree, and expect you to order veggies as side dishes, this is especially true of pasta. You just get pasta, and if you want salad or anything else, you need to order it. Water is generally served in the bottle, and there is a charge for it – but often, restauarnts assume you want it (because all Italians want it) and put it on your table. If you open the bottle, you are charged (it’s reasonable). If you leave it unopened, eventually they will take it away. Specify whether you want still (flat) water “sin gas” or fizzy (sparkling) water “con gas.” There’s nothing wrong with drinking tap water, but it isn’t done so much, and because water is local, the tap water will most likely taste odd to you.
