It’s only like debt for the purposes of government budgeting. For a person, the ‘student loans’ are much more like a graduate tax which is weirdly regressive. The tax is only on monthly earnings above a certain level so lower earners pay a much lower effective rate than the marginal 9% rate. In particular, if you aren’t earning money you don’t pay, when you die your estate doesn’t pay anything, and if you go bankrupt the ‘debt’ is not written off.

£50k is a lot to your sister because of the sorry state of the British jobs market.