Great, Michael! I'm really excited for you. I know the euphoria one can feel when they find information on their family. I'm so glad I was able to help point you in the right direction.
I don't have any records on hand to help you, but I do have some advice.
I took a look at the 1880 census for your Andrews families but could find nothing on them in the 1870 census, at least by those names. Looking at their 1880 neighbors, I followed the ones immediately next to them, Joseph Powell, age 46, to try to locate yours in 1870. But there was no sign of them anywhere in North Carolina, even though Powell was surrounded by some of the same family names as in 1880. The township in Martin County was called Robersonville in 1880 where your family was located, but there was no town by that name in 1870, and Powell was living in Williamston Township. If you don't already know, I would suggest you do a little historical research of the County and see if Robersonville & Williamston were the same places. A Google search should probably come up with something on that. Anyway, it looks like your family may have had different names prior to 1880, so you might want to first take a look at all of the Andrews in Martin County and see if any of their family units matched George Andrews. Even though the names may not match, you may find members with similar names and ages, sometimes even just one person with the same name. If that doesn't work, try a family with the same first names as yours in the township, each time spreading out the area until you've covered the entire County if you can't find them in the township. If your family was living in Martin County in 1870, finding them by this method may be your best bet, even though it may sound like a huge job, you're lucky in that there weren't a whole lot of Andrews in Martin County. Also remember to check out where the neighbors were located in the various years, because many times you'll find yours still living nearby. Once you do find a family you suspect is yours, you would check out there locations in other census years also, to make sure they are the right family. Also, don't expect to find your family with the exact same ages in every census because that won't always be the case, use a little leeway in each direction when trying to determine if someone you think is yours is truly the right one. Also, try variations in the spellings of the names you found for your family, such as you did for Ferebe or Pherebe; try Pheobe, Phebe,Fereby,Ferabe,Phoeby, etc. and for some peculiar reason I've also seen the name as Thilby, and with "Th" in place of "Ph" in the other spellings. Try not to get discouraged, though, you've already proven that it's not hopeless. It just takes a little extra work.