If I may answer, the slaves described belonged to the Custis Estate after Mr Custis died in 1857. A casual reader might assume that the slaves belonged to Robert E Lee, the executor of the estate. It would be helpful to understand that Robert E Lee never owned either a home or a slave, certainly not Arlington or any of the slaves who lived there.

In my opinion, the nps.gov site on slavery at Arlington is reasonable, accurate and fair to all parties mentioned.
http://www.nps.gov/museum/exhibits/arho/slavery.html