John Marshall McMillen's wife was Harriet M. Shepherd who was born 27 JUL 1857 in Iowa; she died 21 OCT 1894 in Iowa. She's buried in IOOF cemetery near her parents in Indianola, Warren County, Iowa.
John and Harriet married in Indianola 27 Feb 1878. They had three children: Henry, Clint, and Mary Francis.
John was born about 1846 in Pennsylvania or Ohio. He died in 1914 in Oklahoma.
Robert McMillen who is also buried in the IOOF cemetery might be John's brother, but I'm not sure. Robert was the son of Dr. C.V. McMillen who is buried in Linn County, Iowa.
I'm wondering if there was an Indianola obituary for Harriet in 1894 (Oct. 21) or an obituary for Robert McMillen who died Jun. 26, 1915. Perhaps one of these obituaries would give some clues about John McMillen and his parents/siblings.