As it is already established that the photo is circa 1930 from the studio evidence, this would make a person born in 1897 in their early 30's, which in my opinion is consistent with the seated lady.
The two girls look to me, to be 20ish +/- 3yrs or so. As the coats are unusual, yet all the same basic style with the large collars and the loose belt with button, it would lead me to think that the 3 women are close, either relatives or friends. I would be exploring the possibility of cousins, if a sister connection has been ruled out.