Hmm ok, I didn't want to become too complicated but I guess I have to.
The differences between ko(no), so(no), and a(no) are ONLY perceived distance. If all the people in the photograph are within the same plane (ie next to each other) then you can't differentiate them with distance words because no one person is further than any other person. If, however, it's a distance shot with an aunt in the foreground and a cousin way over there in the background, then yes you can do that.
Keep in mind Nancy's only in her second semester of Japanese so she hasn't got all that much to work with.
Anyway, the way I'd personally do it is:
"the person on my father's left hand side is his younger sister, and the person on his right hand side is also his younger sister. The one behind him is his younger brother".
I'm not going to translate it though, because that's your job (and I've forgotten how to say "--- hand side").