Wow. That's a lot of driving...for you and for a young pup.
Is there a particular reason you're against having the pup flown to you? A few hour flight is much less stressful on a pup than a two day drive--and there's less risk of disease since pups are required up to date vaccines to fly. Stopping at gas stations and rest areas, etc is incredibly risky for an 8wk puppy whose immune system is somewhat weakened and who is still very susceptible to parvovirus, parasites, and other diseases.
If flying is completely out of the question, then I'd recommend crating the puppy during the trip. Not sure if the breeder has begun crate training or not. If he/she has, this should work well. If not, the puppy is likely to cry and that's not going to be fun on such a long trip. If absolutely necessary, I'd say you could put the puppy on the passenger floorboard space with your brother and yourself taking turns sitting with her between your feet. Obv the puppy will be less likely to get into trouble or use the bathroom in a more confined area. I don't like suggesting this because it's really not safe and you would likely lose the puppy in event of an accident. Also a good idea to bring toys and treats to keep the puppy amused. Please..when you stop for potty breaks--which should be pretty frequent--maybe a couple mins every hour or 90min, stop in an area that is NOT likely to have had other dogs frequenting. Do NOT stop at rest areas, gas stations, fast food restaurants, etc. Stop in areas that you feel pretty confident haven't been used by other dogs. You'll significantly lessen the likelihood of your puppy getting sick...
Hope others have some suggestions for you..
Malachi (Kai) 'My Rescue Angel' Black Male GSD RIP My 'Midnite Rider'
my GSD is smarter than my ex (more handsome, too!)