This appears to be a large family picture to me. The women all appear to be related (three generations). I believe the man on the left was posed by the photographer but would be the husband of the woman on the right, so he would be a son-in-law of your great-grandparents. Does that make sense from your family history? Did your great-grandparents have a daughter who was married and had at least one daughter? Also, I believe that this was probably taken at the time that the girl turned 15. There would have been a religious ceremony and stern warnings to the girl about proper behavior. This would have been before the ultra-extravagant parties (Quinceanera) that 15-year-old Latino girls often have now where they wear long white gowns, etc. That would explain the flowers that the mother and daughter have. I love my idea...hope you do, too!

From what I've seen of old pics I'd say the first man is the son of the woman in the chair. The second man is the woman in the chair's husband. The young girl would be the daughter of the first man & grandchild of the woman in the chair & second man. The last woman would be the first man's wife.
The first man's hand on the one in the chair would be a tip off to the natural affection & respect he holds towards his mother. The child would be before the mother because she's blood kin. But she holds onto her mom's arm to link them all together. Noting she's not standing as closely to the others as she is to her mother I'd guess she feels closer (either consciously or subconsciously)to her than the others, especially her grandparents.
I honestly don't think it would be her brother though. In all the older ones I've seen with siblings brothers & sisters weren't touching as that wasn't considered proper until the mid 1900s. If you look at old pics of siblings together they're never touching, even when one is directly over top the other.

Hubby who is a little older than me said the only other possibility was the second woman was a daughter also. But if that was the case the second woman would be beside the second man with the daughter standing in front between them or seated beside the grandmother.
So he agrees the scenario I gave was the most likely.

I believe the younger man on the left is the son of the older couple because he has eyes that are similar to the older woman and a nose that is similar to the older man. The young girl has a nose similar to the woman on the far right, therefore I believe she is her daughter, but her eyes look like the man on the far left and the older woman. I think she is the granddaughter of the older couple.

I agree with the post from maryrenna that this is a three generation photo. Son and daughter of the parents on either each end of the picture and the young girl is the granddaughter. Facial characteristics between the son and father, especially the ears is striking. The son has a more direct pose to the photographer than the father, but you can still see the same shape. The eyes and nose of the grandmother and her daughter are very similar. As others have said the hand wrapped around the arm could very well indicate mother daughter as the facial characteristics again are similar. It may be the son-in-law is deceased or just not present for the photo.

I think the gentleman on the end is the son of the older couple with his wife and daughter on the other end

After looking at several old pictures of my ancestors, I believe they are a father & mother with their three children. Many times children were spaced far apart, due to deaths or a last child being born some years after the older ones.

The woman on the far right bears such a strong facial resemblance to both your great grandfather and your great grandmother that I'm sure she must be their daughter. She has the eyes and distinctive nose shape of your great grandmother and the mouth shape of your great grandfather.

I just happened to stumble on your question and feel pretty strong about the following (even tho I have no italian blood or a sense of heirarchy for this other than common sense). The old couple seated are obviously the main subject (your great grandparents?). To their left is their son whose nose and hairline greatly resembles his father's. Off to the right is the son's wife and daughter who were permitted to be photo's with the rest but clearly not part of the inner circle.