It does not make any sense. Let's say the sister is a master student, which typically a two-year program (sometimes, it is a year), the brother is undergraduate student, he will finish 50% of his hours by the time she graduates. Obviously, whomever put this rule did not think this through. I suggest you re-request for a scholarship separation and detail your situation (that's impossible for 75% to be achieved for a number of reasons "explain them"). Send an email to your supervisor as well.