the soulfulMofo knows what he is talking about. Good choices! Though I personally find J5 to be a bit overrated.. still definitely quality though!
I've been getting into hip hop a LOT these days, so I consider myself to have a fairly good idea of what i'm talking about.
Now here are some songs you HAVE to get for her sake, and yours (because they are just that good, you see):
Talib Kweli - Get By (you have to be cold and dead to dislike this song. seriously. everyone loves it)
Roots Manuva - Clockwork
Aesop Rock - Daylight
Bomb the Bass - Bug Powder Dust (you might have trouble finding the original version, let me know if you need help, but don't pm me cause i'm not a premie)
Latyrx - Storm Warning
Blackalicious - Deception, Smithzonian Institute of Rhyme
Outkast - ATLiens
Roots - Thought at Work
Roots w/ Erika Badu - You got me
Jurassic 5 - The Influence
El-P - Stepfather Factory
Prefuse 73 - Plastic
Dilated Peoples - Worst Comes to Worst
Deltron 3030 - Positive Contact/3030
Jeru the Damaja - Can't Stop the Prophet (i prefer the 5 minute version)
Prefuse 73, Mos Def, Diverse - Wylin' Out (rjd2 remix)
Common - I am Music
Cinematic Orchestra - All Things to All Men ft/ Roots Manuva (your sister prolly won't like this one, but it's awwweesome, and I think you are a man of good taste so you probably will)
you can't do too bad trying to hawk Talib Kweli .. his album Quality is rather amazing, accessible. It's happy hip hop, really quite worthy of much praise imo. read the pitchfork review, they got it right. Anything you get off it will probably be catchy and intelligent and of very high quality.
Outkast is a good example of popular hip hop that is mainstream and still ubergood. So Fresh So Clean, Wheelz of Steel, Rosa Parks, 13th Floor, Jazzy Belle, Spottieottiedopaliscious, and Bombs Over Baghdad are some excellent ones i didn't mention up there. And of course Hey Ya, but that's not really hip hop. ATLiens (the album) is probably my alltime favourite album right now, it's really a work of pure brilliance in my opinion.. every single track is tight, deep, and smooth.
The Roots are very very good. all of the stuff off their new album Phrenology is accessable and very high quality... try Sacrifice (with nelly furtado), Rolling with the heat (with Talib Kweli), and the above mentioned Thought at Work.
Now I dunno how old your sister is, but some of these songs employ the old cusswords. etc. I think you are doing a good deed, it's always possible to find real high quality music that appeals to even the mainstream stickers.
If you have any other questions, I am happy to be your hip hop slave.
Hieroglyphics - Maggie May is real good too.