The mothers have a pair of X-Chromosomes. So they do not paly significant role in determining the sex of a new born.
All children will inherit an X-chromosomes from their mother regardless whether they are boys or girls. Therefore, sex is always determined by the other sex choromosome that they inherits from the father. One who inherits teh X-chromosomes of father wilbe girl while who inherits Y-chromosome of the father will be boy.