Thanks to a generous ancestry.com member, I now have more information on the connection between the Weeks sisters and Grandmother Moffatt:

“Grandmother Moffatt” is Henrietta Shearer (1829-1906) who married John Henry Moffatt (1829-1864) in Scotland, moved to PA, (where he was a house builder) and bore him five children. After his death in 1864, she married his cousin James W. Moffatt and bore him three more children, the youngest of whom was Marion “Effie” Moffatt (1876- ), who married Herman T. Weeks (1864- ) and is the mother of the “Weeks Sisters” Anna Y Weeks (1902-1937) and Isabel C. Weeks (1907-1999). She also had a son, (their brother) Frank H. Weeks (1904-1971).

Thanks Bart for this great information!