Is it obligatory to return money taken from a maternal aunt without her knowledge, considering that she is generally forgiving? And can the repayment be postponed until money is available? And is it obligatory to return the money with an increase if years have passed since it was taken?

There is no doubt that taking money from one's aunt without her knowledge is a reprehensible act. And since the right belongs to a human being, repentance from it necessitates either returning the right or seeking forgiveness. You have two options: either ask your aunt to absolve you of the money after explicitly informing her of the truth of the matter, or return what you took to her without telling her that you stole it, and when you are able to repay it, you should promptly do so.