Colour blind man has one X chromosome with the gene for colour blindness which he receives from his mother. His colour blind sister has two X chromosomes each with the gene for colour blindness. She has received one of these from her father and one from her mother. Thus from this information we can conclude that their father is colour blind and their mother has at least one X chromosome for colour blindness. Also they have a normal brother. That means their mother has one X chromosome without colour blindness genes. Hence mother is a carrier.