I would think the opposite... I dont think it would help get an interview... what are you going to write? Write it on your resume? They probably wouldnt even know you're the cousin of someone who went there.
But I think if you were in the interview and said "I know a lot about the program and strengths etc because my cousin was XXX" then I think it will help you as it will just make you easier to remember. I think that's half the battle in a residency interview is being remembered - all the applicants become a blue and only a few stand out for whatever reason