Hi there,
The sex of new born kittens is random and there are no vitamins or tricks to increase the chances of getting a certain sex.
Due to the kitten sex being random there are times that it may appear you get more boys than girls, times when you may get more girls than boys, and times where it is more of an even mix of boys and girls.
I hope this clarifies things, if you have any other questions, please let me know