@zahmata Stanford has extremely generous (some would say uniquely generous) financial aid. In recent years, only Harvard and Princeton have offered similar need-based aid packages. So no, that projected 30k is not something that you would be able to use to negotiate a better package elsewhere. And frankly, why would you want to? If you would make the cut at Stanford, and you could afford that 30k, then I expect you would just grab it instead of a peer institution at 60k.
The financial aid formulas are heavily weighted to income. Your parents’ 300k income is almost certainly the primary cause of the 60k EFC that you have been getting. Depending on how your sister’s college fund is held, if she doesn’t use all of it up, it might be possible for your parents to use the remainder for you. If it is in a 529 fund, that is an easy switch. So find out about that money when you talk with your parents.
If you like Spelman, you could take a look at some of the other women’s colleges. Bryn Mawr and Mt. Holyoke do have some merit-based scholarships, but I don’t know if they can bring the cost down to the range that you need.
Once you do have a “my parents can pay X” figure, you might want to take some of your questions to the Financial Aid Forum. The experts there may have a few more ideas for you that have been missed here.