It seems to me like you are talking about a couple different things here:

1) She doesn't 'get a say'. Of course she gets a say. Her sister comes to her and talks to her about her career choice. Is the LW supposed to stand there mute?

2) She doesn't get to make the actual choice. Of course not, I'm not quite sure how somebody could actually force another person's career - short of being somebody like a mentor or something.

3) Your sister guilting and manipulating you. Horrible thing for her to do, doesn't matter as to the why. Not really the dynamic being talked about here. You'll note the LW is convinced her sister will be wildly successful - not telling her sister she will fail.

4) What you didn't talk about, but to me was the most interesting thing, is that this is a pattern arising out of an abusive (or at least very rough) childhood for both of them. Odds are quite good that neither of them are in a healthy pattern. They do need to 'let go' and start relating to each other as healthy adults but odds are that is going to take some serious intervention by trained professionals. Just because you may think your sister is a spoiled brat that could stop at any time, doesn't mean that these people don't have deeper (and harder to resolve) problems.