If X has two sisters - Y and Z, and W’s motheris the sister of Y’s father, then W is Z’s ...............

here we are representing male by $$'+'$$ and female by $$'-'$$

brother- sister relation by $$\rightarrow$$

parents and children relation by $$\downarrow$$

now according to first statement, we have

$$X$$ $$\rightarrow$$ $$Y^-$$ $$\rightarrow$$ $$Z^-$$

according to second statement, we have

              $$\Box^+$$   $$\rightarrow$$       $$\Box^-$$

              $$\downarrow$$                    $$\downarrow$$

$$X$$ $$\rightarrow$$ $$Y^-$$ $$\rightarrow$$ $$Z^-$$    $$W$$

Thus W is Z's Cousin.