Ok, first off, Combat Casting is a horrible choice, except as a pclass prereq. Skill Focus (Concentration) is much more useful.
The instruction here was "kicks butt" which is a bit vague, so I'll offer some options, up to about 6th level. (You should have a good idea what you want to do by that point.)
High elf is your best choice of subrace (that's the basic, PHB elf)
Living Greyhawk has 28 point buy, so I'd recommend a stat distribution something like:
Str 8, Dex 14, Con 12, Int 10, Wis 8, Cha 18. For a generic sorcerer build.
For a more focused damage dealing build, go instead with:
Str 8, Dex 18, Con 12, Int 10, Wis 8, Cha 16 (this will allow you to take advantage of all sorts of ray and orb spells).
Spell choices for a generalist build - go for a mix of offense, defense, buff and debuff. Don't worry too much about utility spells, they don't come up that often for Living Greyhawk.
1st level - color spray, mage armor, magic missile, ray of enfeeblement, all your standard 1st level spells are nice. A surprising choice for LG is endure elements - every so often you hit an adventure that tries to freeze or broil your party, and when you need it, everybody in the party needs it. Nice for sorcerers, but probably not what you want to spend one of your first two spell choices on.
2nd level - Glitterdust, false life, scorching ray, glitterdust, see invisibility, glitterdust, and more glitterdust.
3rd level - Haste, fireball, dispel magic - all the standards.
For a blaster build (substitute for some of the above), looking for more variety in damage dealing, so you always have the right attack spell for the target. Don't neglect defense, but you can let buff spells slide (you'll probably still want a few debuffs though, since you'll occasionally run into things that your direct attacks aren't optimal for attacking).
1st - Lesser orb spells (from complete arcane) - you can't take them at 1st level, but you can get them at 3rd level, when you learn a new 1st level spell. With this build, you may want to take magic missile at first level, even though it's not very useful until later, so that you can take the orb spell of choice at 3rd level.
2nd - Scorching Ray is the staple here.
3rd - Fireball is the damage spell of choice, but don't overlook stinking cloud for making the DM really miserable.
Feats:
Generalist - Spell Focus, Greater Spell Focus, Leadership (alternately Craft Wondrous Item for one of those, especially if you always play with a full table of 6 players where Leadership isn't useful). Pick your favorite school and get really good at it.
Blaster - Point Blank Shot, Precise Shot, Empower Spell (at 9th level - Sculpt Spell).
Skills:
Max Concentration. Don't worry about anything else (you could even drop Int to 8 if your sister doesn't mind playing a bit of a dumb character). Sorcerers only need skill points if they're planning on hitting a prestige class, so anything after Concentration is bonus.
