I understand. You want to know how much fin aid you might get from the boarding school, depending upon whether or not your sibling’s college tuition is high or low.
So if your parents make 300K, they’re not going to qualify for fin aid for the child in college. Let’s assume they’re full pay for UT Austin, at 55K/yr total cost. Now you’re looking at boarding school tuition. another 55K/yr or more. I suspect that when you apply, you will explain to them the financial situation of the family, and they will look at whether or not to give you aid, with the award modified if your brother decides to attend UT Austin. You could contact the school assuming you get in, and ask if they will modify their fin aid offer based upon what your brother’s college expense is, as soon as he decides where he will go.