Wow, you know what they say about assumptions... Well, I had assumed that Harriet was the Aunt of Margaret, but you have made me see that it's possible the person who wrote the names on the reverse could have been the niece or nephew by marriage of Harriet, whose younger sister Margaret just happened to be in the photograph.

Thanks for your help! Do you mind if I add your info to the blog post in case someone else happens upon the photograph?
Pam