I know legacy usually applies to ‘parents’, but can legacy extend to uncles/aunts, siblings, grandparents (without parents), etc., in terms of BS admissions? Also, will major donations/famous names change that?
There is no box on the application that says “Check here if you are legacy.” You answer the questions and schools will do with it what it will. Any school is free to define legacy as it sees fit, and to decide how much of a bump, if any, legacy will have on an application. The usual definition is parent. Some schools include grandparents and/or siblings. There is no standard definition that applies to all cases.
Similarly, major donations in the 8 figure range and/or celebrities may provide a boost to some schools. However, for the most selective schools, any applicant with a hook will still need stats that are somewhat within range of the average.
Note however that should one make the only large donation in the same year as the application, one should not expect a quid pro quo, The school will be more than happy to take the money, name a building, and still issue a rejection letter.