J.L.
I drove all the time with my newborn. I did about a 7 hour trip 2-3 times a month, since she was 2 weeks old - 8 months old. My mom was ill and I had to constantly drive to my old home town to visit her. Honestly, if I had the money I would have flown with the baby, way easier, was shorter trip.
I had to pull over all the time to get her out of the car seat so she could stretch out, change her, and feed her. It made the trip longer, although with the help of your sister it might be easier for you. I brought my little cousin with me a few times and she was such a help feeding the baby her bottle while I drove. It was one less thing we had to pull over for. I would still go see your sister and your new nephew no matter how you get there. Plus at that age, they sleep a lot in the car. =)
Good Luck!