Well that response was pretty vague. Check out
VectorLinux. I use it on the laptop below and it works great. The newest version (4.3) is based on Slackware 9.1, but is much easier to use and set-up. As far as the GUI goes it includes 3, FluxBox, a modified IceWM, and XFCE. For your sister I would recommend the modified IceWM. It's modified to support icons (now that I think about it so is the FluxBox) and it looks just like Windows. In fact if you want it to it has a couple of windows xp themes with the infamous start button. It also has a theme call sortofaqua (my personal favorite) that looks like the OSX GUI without the fancy animations. It includes it's own package system but includes the slackware package system and I haven't had any trouble installing slackware pakages (beyond dependency problems).
As far as software goes, for internet it has the Mozilla Suite for web browsing and e-mail or for something lighter it has Dillo for web browsing and sylpheed for e-mail. I would strongly recommend getting rid of Mozilla and downloading Firefox because it's a lot lighter (not that I'm partial, my signature proves that).
For office programs it has abiword for word processing, which can read and write to windows office format or its own, and ABS for spreadsheets and I believe it can do the same.
For multimedia it has XMMS and Mplayer. Also, for some strange reason it has, with the alsamixer, 3 mixer programs.
It also has a couple games, Lbreakout, a Break out clone, and Super Tux, a Mario Brothers clone featuring every geeks favorite penguin.
It also has a program called VASM (can't remember what it stands for) with a bunch of tools in it that will run in both text and graphical mode
There's a bunch of other stuff but I can't think of what it is.
I will warn you that the most intimidating part is that it doesn't boot to X by default. You have to login in text mode then type startx. There is an option in VASM to make it boot into X but I'm not sure how well it'll work since I've never used it.
This is the machine that I've learned most of Linux on and I personally think it's just as easy to use as any of the major user-friendly distros (mandrake, fedora, et.).
a couple last notes I forgot is that it use Kernel version 2.6.7, and for audio it uses ALSA. I've never had any problem with ALSA configuring the sound, but on some systems it defaults to mute for some reason.
EDIT: It requires 835MB to install and at least 64MB for Swap. Usually you need a boot partition but if Vector is the only OS on the system I found that if you put the / (root) partition in the front and select it as bootable you won't need the boot partition, this is what I do and it works just fine (just thought I'd save you a little space).