Eh, good question! DCDA twins (dichorionic diamniotic, the non-identical ones) can definitely not know they're twins until the scan lah, especially if the parents haven't told them yet or if they grew up apart. A few of my mum friends have DCDA twins, and honestly some of them only found out later in life when they did DNA tests or family talks became more open. It really depends on the family situation and whether the mum shared this info with them growing up or not.