Sitting down, and crossing your legs won’t cause varicose or spider veins.
But, there are things that will cause varicose and spider veins:
genetics, standing a lot, pregnancy, or trauma
As with most beauty traits, unsightly veins can be inherited.
Standing for prolonged time periods cause the vascular system to work extra hard pumping blood from the legs to the heart. Pregnancy can also put added pressure on the circulatory system, and experiencing trauma (getting hit with a softball for example), can also lead to varicose veins