I make a up a scenario as if it were a problem, the steps to solving this scenario are below:
Step 1: Set up your problem. 1/2 of 1/3 = 1/2 x 1/3. You will notice that your problem has unlike denominators, meaning the bottom numbers are different from each other. Although this is important when adding or subtracting fractions, it is not a problem when multiplying.
Step 2: Multiply the top numbers, called the numerators. 1 x 1 = 1. This is the numerator for your answer.
Step 3: Multiply the denominators. 2 x 3 = 6. This is the denominator for your answer.
Step 4: Put your answer together: 1/6. Your brother and sister will end up with 1/6 of the pizza each!