Mary J Duffy and John Francis Duffy (died 1953) are the children of John Martin Duffy (died 1962) and his wife Harriett E. M. Zimmerman. You can find them all on Findagrave along with their baby sister Amelia Josephine (1904-1904) in the Saint Joseph Cemetery, Keene, NH. The tombstone lists all their names (including Harriett's maiden name of Zimmerman).
Mary J Duffy was also listed in the 1940 census in same State Hospital as her brother John Francis Duffy. And they are listed together in several censuses for further proof belonged to this family.
Hope this helps for dates and your proof that they are the siblings of Harriet E Duffy Breen.