I suggest Ouran Highschool Host Club. It's goofy and the dub is just fantastic.
Aside from that? Let me look at my collection real quick.
Uhh...
Evangelion because depression and giant robots
K-On! because it's fucking adorable and really doesn't have a plot other than be cute and drink tea.
Nichijou because it's cute like K-On! and funny as all hell like Ouran Host Club. One of my personal favorites.
Fullmetal Alchemist: Brotherhood because it's a classic and has a great story with well balanced humorous and serious moments. Might be a bit long for someone just starting anime, though.
For suggestions that aren't necessarily for your sister:
FLCL - Bullshitly stupid and quite funny. An animation spectacular that lasts six episodes.
Gurren Lagann - More bullshitly stupid stuff, but awesomely action-tastic. It's amazing and is something I think people of all ages can enjoy, simply because it has enough saturday-morning cartoon action for the kids and enough plot and silly moments for the adults. The dub is also the best ever. Much better than Cowboy Bebop's. I said it.
Panty & Stocking With Garterbelt - Yeaaahhhh, not something you want to watch with your sister. Think two angels are banned from heaven - one for being a slut, the other for being a fatass that eats all of the food - and they have to kill ghosts to buy their way back into heaven by using their weapons of namesake (Panties and stockings) to vanquish said ghosts.
Baccano! - A story where everyone is the main character. 1930s immortal gangsters, thieves, mass murders, and so on. It's the show that got me back into watching anime.
I dunno what else. Good luck.