Hi Sharon!
It was so kind of you to purchase
Walt Disney World Resort theme park tickets for your parents! We enjoyed a trip to the "Most Magical Place on Earth" with my parents this past winter and it was so much fun to enjoy the theme parks with multiple generations of my family.
Walt Disney World Resort theme park tickets can be reassigned to other Guests as long as no admission days have been used. Once tickets have been used to enjoy the theme parks, they cannot be reassigned or used by a different Guest. So your brother is welcome to use one of the theme park tickets you originally purchased for your parents as long as they have not already been used by your parents. However, keep in mind that once he uses a ticket to visit the theme parks for two days, your parents won't be able to use the remaining two admission days.
Sharon, if it's a possibility, your parents can consider visiting the theme parks for partial days so they can still use all four of the admissions on their theme park tickets. In my experience traveling with my parents and my grandparents, they often enjoy visiting the theme parks for select portions of the day, allowing them plenty of time to relax in our Disney Resort hotel while my husband, my kids, and I experience the rest of the activities we would like to enjoy in the theme parks. So your parents may still be able to use all four days of their theme park tickets while taking their time enjoying the Disney magic!
Come back to see us if we can help you again as you continue planning your trip. T-T-F-N, ta ta for now!
~Crystal