A simple signed (and preferably witnessed) document would suffice to prove that the money was a loan, rather than a gift:
"I acknowledge receipt of the sum of forty thousands pounds from my sister Susan as an interest-free loan loan to be repaid upon the sale of my property at 42a Acacia Avenue, Little Boggit, Hampshire"
As long as it's clear that the money is a loan then it remains part of your estate for Inheritance Tax purposes if you were to die before it was repaid (as it would anyway, even if it was a gift, if you were to die within 7 years). As there's no interest, the loan doesn't affect either your income or that of your sister, so there would seem to be no implications with regard to Income Tax. Similarly, as there's no 'gain', I can't see that Capital Gains Tax would apply. Therefore I assume that HMRC would have no interest whatsoever in the loan.