I don't know why GMM puts Sanwan at the end in the poster but the synopsis said "It is focused on Sanrak and Sandee's OLDER sisters" so Sanwan is older than them. Another point, Sandee is a freshman and Sanwan is clearly not a freshman (in the first/second ep, I think)
Fun fact, based on the actresses' profiles, the rank from the oldest to youngest is Sansuay - Sanwan - Sandee - Sanson - Sanrak
Fun fact, based on the actresses' profiles, the rank from the oldest to youngest is Sansuay - Sanwan - Sandee - Sanson - Sanrak