The only person who looks blameless in this story is the godmother.
Your sister doesn't really have any right to be outraged by your parents decision, especially after inheriting so much money unexpectedly. Moreover, it is your parents' legitimate right to decide, as what they do with their bequest is their business. Unless.....there were already some pre-existing promise in place that their estate would be divided equally between their children.
I don't feel that you have any right to feel shortchanged if your parents decide to leave their estate equally to you and your sister - after all, what your sister's godparent did has nothing to do with anyone other than the godparent and the godchild. Moreover, you have said you would still get "a seven figure sum". That is a serious amount of money - and it makes you sound rather self-entitled, in the context of your feeling that something unfair has happened.
Your parents are possibly at fault, because if there has always been either a formal or implicit understanding that their estate would be divided equally between you two sisters, then to suddenly change that because of an ex-familia accident of fortune in favour of your sister would seem unfair to a degree.
But as someone has already pointed out, it is currently still your parents' money - so they can do whatever they want with it. Whilst you explain that your sister is a nurse (overworked, underpaid national treasure), you don't tell us what you do. You could be independently wealthy already - or happily married to someone who is. Maybe just accept that your sister may/will always be better off than you, but that you have both done pretty well, in part thanks to the efforts and wealth of other people.
Let that sink in for a while.