To me, the 3 ladies look alike the one seated would be grandmother, far right would be daughter, 2nd from right would be granddaughter. The older man is the husband of the older lady. I think the man on the left is the son of the older couple. There are no in laws in the photo.
Coming from an Italian family I have found that in group photos the oldest couple is placed together, their child or children are next to them with the child or children husband or wife next, then come grandchildren.