Phuean Phaeng

A love triangle between Lor and the two daughters of Por Pit named Phuean and Phaeng. Phuean is the selfish older sister and Phaeng is the tomboy younger sister. Lor is Por Pit's distanced nephew. He was taken in by Por Pit after his parents died. Phuean, Phaeng, and Lor were raised up together. Lor took care of Phaeng since they're little because Phaeng was very attached to him.
But when they got older Lor fell for Phuean and the two got engaged. Lor loved Phuean and saw Phaeng as an annoying younger sister. He promised to love Phuean forever and if he has another woman he shall die for betraying their love.
Then Lor fell ill with malaria so he was taken to the city to be treated. Phuean and Phaeng went with him. While they lived in the city Phuean had an affair with a rich man and left Phaeng to care for the sick Lor.
(Source: iheartthailakronandsouthasiadrama.blogspot.com)