Wife and husband?
Sister and brother?
Today's mystery real photo postcard gives us no clues (that I'm aware of) regarding the relationship between these two young individuals.
If you forced me to hazard a guess, I would say it's a studio portrait of a Mennonite couple on their wedding day. (Maybe it was part of the photographer's package deal, and they didn't have anyone to mail the postcard to.)
The particulars of the AZO stamp box on the back, with two triangle pointing upward and two pointing downward, set the date range for this photograph between 1910 and 1930. There is no writing on the back of the card.