You've gotten some interesting information here, most of it true, but some of it's confusing. I've been into A/V for over 20 years (my personal home theater was featured in Electronic House a few months ago) and can offer a little advice that I hope you find helpful.
What does your father-in-law currently have?
You do need an amplifier of some kind. However, as rybitski said, most stereos have them built in. Specifically, receivers do, but pre-amps do not. If your father-in-law currently has a nicer receiver, there is a chance that it has a powered zone output that would work. How much power do you need? The rated power output doesn't matter much as long as you have a decent receiver/amplifier. Speakers handle fairly high amounts of clean power well. However, they do not handle distortion very well at all. You have more of a chance of killing your speakers with a 10 watt no-name brand amp than you do with a 100 watt Krell amp. In either case, you just want to make sure that you don't turn it up too loud.
You also need a source for the music. This could simply be the built-in radio tuner in a receiver or it could be a CD player, ipod doc, etc. (or all the above).
Lastly, you need speaker wire to connect the speakers to the receiver/amplifier. 14 gauge should be plenty unless you are running a long distance (several hundred feet). If you want it to look good, installing the speaker cable will likely be the hardest part of it all. If you can give me some specifics on where things are located, I coudl try to give you some thoughts on that part as well.
CJ