My niece graduated 2 years ago and feels she is underpaid how can she ask for more?

I would need more context in this. How underpaid is she and on what basis has she arrived at this conclusion?

For example, is she underpaid compared to her peers in the same department or in another bank? Or is it based on the latest salary surveys or websites like Glassdoor?
Has she spoken to headhunters and has she been approached for opportunities that pays more?

Once she has determined that she is indeed underpaid, she can do some of these to ask for more:

• find the right timing to discuss a salary increase eg if the local bank is currently downsizing or retrenching people in her team it would not be the right time

• gather evidence that she is underpaid and deserves more (see above)

• come up with some facts and figures eg she has been given new responsibilities since her last increment

• use a hypothetical question to start the conversation with the boss instead of a downright statement asking for a raise. Eg if I would like to review my salary, what has to happen before I can get a raise?

Hope that helps! :)