The only times it seems to have a negative impact are when applicants use it poorly. Like, say, all of their clinical experience is working with their parents. Or letters are written by a parent/close friend, rather than someone who knows them through their own work. Or their personal statement centers on what they saw their parents do.
Just having parents who are doctors isn't an issue, from the applicants I've seen go through. It's not having developed experiences outside of familial connections.