Applying JHU RD with double legacy?

Hello, my little brother is considering JHU, among many other top schools for college. His credentials are as follows:

SAT: 2300/2400
SAT2 Math2: 800; Biology: 750, Physics: 750
uGPA: 4.0
wGPA: 4.75
15 AP classes
Varsity Cross Country 4 years (currently captain)
Varsity Track 3 years (captain last year)
2 NIH Summer Internships in Biomedical Research

He’s a great applicant but considering the recent competitiveness of the selection process at top 20 schools, whether or not he gets admitted to a top school over a similar applicant will most likely be decided by chance. However, both my father and I attended Johns Hopkins so my brother should be considered a “double legacy.” I feel confident that if he were to apply JHU ED, there is a very high chance we will be admitted. However, if he applies RD, will the double legacy still be helpful? It is my understanding that schools admit legacy applicants b/c they are statistically appeasable for the school’s matriculation/acceptance ratio. However, if a legacy applicant were to RD, how would this affect the chances of admission?