Does a sibling who graduated from the college register as a legacy? Does a sibling who currently go to the college work as a legacy? Do they both help your chances of getting in? (Harvard, Upenn, Williams, Bowdoin)
Legacy, especially at top schools that you list, means parents and sometimes grandparents.
Siblings don’t produce legacy status, but in my experience, students who have a sibling who attended and did very well have a higher chance of being admitted.
Every college counts legacy differently. You need to look at the policy for that particular school.