From a long time, it was believed that sitting with your legs crossed leads to varicose veins, but that is not true.
Varicose veins are caused when there are some complications within the valves in your veins and it has to work harder to pump blood towards the heart. In this condition, the blood gets collected and cause bulging in the veins. People who either keep sitting or standing for a very long are at the risk of developing Varicose veins. It has nothing to do with your sitting posture.