I am from Sweden, now living in USA
1. Sweden is safe, much safer than any place in the USA (people don't have guns only Police and a few people in sport shooting clubs (like people training for the olympics, etc))
2. Stockholm in winter is awesome
3. You will be able to find tons of American junk food, also department stores have cafes where you can look at foods before you buy. My sister-in-law is a crazy picky eater and a teenager. She eats just fine when she travels to Sweden with me. She also likes to go to the grocery store and buy the items she wants, and bring them back to our hotel room. I am not from Stockholm, but when I go with her to Sweden I always make certain to spend a few days in Stockholm.
4. Everyone under 80 speaks English and all the tourist maps are available in English. If you try a word or two in Swedish, you will get big points
Some practical things. There is an awesome train that goes from the airport to the city center. I recommend taking this train, and then transferring to a subway. I send all of my husband's American friends this way and they save a ton of money and get to their hotels faster than taking a car. You can walk or take the subway everywhere you will need to go in Stockholm (well you may need to take a ferry to the Vasa Museum) you have no need for cabs (expensive) or a rental car (expensive and taxed daily for being in the city center).
Also Sweden is pretty expensive, and the dollar is week. So sight seeing at museums is pretty reasonable; however shopping and eating are a bit pricey. (Remember you can get some of your tax back if you purchase something and remove it from the country--I can go into more detail later) If you can get free hotel (which will include a huge breakfast) and your dinner meal taken care of by your mother's company you should be all set. Maybe just do a pastry and coffee instead of a lunch.
Okay I don't know where you live right now, but you will definitely need winter boots, a head covering (I wear burberry ear muffs) a scarf, a good coat, and warm gloves. Each store you will go into has a grate (and sometimes a small oil lamp burning in the darkness) you will want to clean your shoes on the grate upon entering the store. For some reason I had to explain that to my husband the first time, it seems pretty logical to me.....
Also because of the location of Sweden on the planet, winter is pretty dark. It is not dark like night, but it really never gets very bright during the day sort of an overcast. Don't be alarmed, it is perfectly normal. You should see the tourists in summer when they figure out we have 20+ hours of sunlight!
Once you know your hotel you can send me a PM and I can give you more specific information. I can tell you tons of stuff to do and the best ways to travel.......heck I could even send you with a 10 page shopping list
I can even loan you a beginners Swedish language CD you can burn to your I pod for the flight.
BTW, my name is Ericka