According to my volume of Descendants of Simon Mellen, c. 1636-1694, pages 195-196, two sisters were born within 8 months of each other. Sarah Agnes Mellen was born 13 November 1869 and Nettie Christina Mellen was born 15 July 1870.
There is an incorrect date for one of the two.
The source I gave, Cutter’s Genealogical and Personal Memoirs Relating to the Families of the State of Massachusetts, volume 4, page 2171, shows the dates given above.
Checking the Massachusetts vital records on the NEHGS website, americanancestors.org, for Cambridge, I find that an unnamed female surnamed Mellen was born on 20 November 1869. This is likely to be Sarah Agnes Mellen.
Nettie Christina Mellen, on the other hand, according to the NEHGS records, was born in July 1871, and died 14 July 1872, aged one year. There was no day given in the date field of the birth record.