Your sister's step daughter sounds just like my son! :) He does not like working on computers (unless it's games), was doing terribly in public school (he's doing excellent now!) and since I work part time during the day, he works pretty much on his own. And he's in the 8th grade. :) We use Seascape Private School's homeschooling curriculum and they have worked out perfect for us. They send everything along with a schedule so he knows specifically what he needs to do every day. If he has any questions he just emails his advisor and he gets the help he needs. It's just been so easy on us both and he really likes his curriculum. He actually thanks me all the time for taking him out of ps and enrolling him in this program. I think he also really likes that he doesn't feel pressured and he gets positive feedback from his teacher. I think Seascape is pretty much the lowest priced full program I've seen, especially with everything that they offer.
Anyways, here's their web address if you need more info: www.seascapecenter.com