If you sister is a registered nurse in the UK and she paid for your course, then it should be quite easy to clear up that point. The only problem that arises then is, if she has only paid for a part of your intended course, ie, 1 year of a 3 or 4 year course. If that is the case, they will probably want to establish that she has the funds and is prepared to pay for you to complete the course.
They probably see it as a possible loophole by which people may be able to enter the country with no intention of completing their chosen course and in fact, every intention of overstaying and working illegally.
Iain.