Unlikely. Most schools specifically ask if you've attended their division- eg, most of my medical school secondaries asked if I had relatives that attended the medical school, but didn't ask about any other school in the university. It's a little more likely that they'd ask about University graduates overall at the undergrad level, but the "sibling effect" is going to be small, especially if the medical school is in a different part of campus. Based on my experiences in undergradaute admissions it's roughly:
Having 2 parents that graduated from the undergrad > Having 1 parent that did > Having a sibling that graduated (or is attending) the undergrad > Having parents that went to graduate divisions > Having a sibling that went (or is attending) the graduate divisions
So if they even ASK about it, it would only be a very, very small factor.
Legacy status isn't a binary thing- there are gradations taken into account.