It differs from school to school, but it will almost never make up for not having the stats. But it can help to get you selected once you have made the initial cut.
If your parent is a major donor and/or is involved (on the board, runs fundraising for his/her class), that is often more powerful than simply having graduated and sent $100 every year.
At many schools, the legacy boost only comes in the ED round. They aren’t going to give you an advantage if they aren’t your first choice.
To get a good answer, you would need to disclose the school and the details of your legacy.