Because twins are very sensitive to each other.
my sister and I are twins and we both are very close and we share everything, we are usually the first ones to know anything about each other. Twins are known to be very connected to others and spirits. Some where I know that salmon are tied in to twins, Im not 100% how but I remember how it works. Also my dad knew that he was having twins, in my culture if you have two swirls in your hair closer to the back of the head that tells you that you would be having twins.