I see the one 1880 Census says that a Rebecca is the sister of Warren Holland and that she is 21 years of age.
The other 1880 census shows a Rebecca, age 29, with her husband Jonathan Reeves, and a young child.
Questions....quite an age difference (8 years) between the two Rebecca's, cant see her being confused about her age if given on two census in the same year. Is Warren Holland actually a brother of your Rebecca? Can you place her family in that location? I can't see Rebecca leaving her young child behind if she was visiting family.
I am inclined to think they are different people.