My feelings are in line with lill on this one, but anything like this is such an individual thing, you just have to do what feels right to you.
Such a horrid thing for the aunt to do. Not, as you say, the money itself but to have so little feeling for her mother's wishes that she would fail to distribute the money as bequeathed. I hope she can sleep at nights. Scratch that, maybe I hope that she can't!
I have recently been getting my own Will done, and have chosen as executors friends who I know would try to do what it was that I wanted, irrespective of their own feelings. From what I have seen, using family members as executors nearly always causes rifts or at least ructions. I'd rather leave the lot to a cats' home than have that as my legacy!
Sabrina, whatever you decide, know that you are a right-thinking, right-acting person, and you are better than those relatives who have acted with so little feeling for their dead mother / grandmother. They will have to live with the knowlege of what they have done; the money they held onto for themselves, that they stole from you and your siblings, will erode their peace of mind. You have something more precious than money and they cannot take that from you.