I am just guessing here but I think that this is a portrait of Adele (Charbonneau) Greenwood and four of her five children. Number 1 is Adele. Number 5 is her youngest child, Arthur. Number 3 is middle child, George. Number 2 (standing) is my great grandmother Minnie or Mary (same person, what you call her depends upon whether you know her from documents or stories). Number 4 (seated) is Adele’s youngest daughter, Eugenie.
I imagine that the occasion was Adele’s wedding to Gilbert Boucher in 1899 — and that the portrait was taken in Pittsfield, MA. Her absent eldest son, Edward A. Greenwood, had already married Julia Gironard, and was living in Manchester, New Hampshire.