Thanks for your response
Yes you will fall into being liable to at least 40% tax on this interest element - so on £2700 - you will pay £1080 tax, and this will apply if you are resident in the UK at that time. (if your income is in excess of £150,000 then 50% tax will apply so £1350 tax)
But if you are still not resident, and your sister is UK resident, then this payment will still be subject to UK tax consideration (as this is where it will arise from) but as a UK citizen you will have your personal tax allowances, to cover this payment, which will then render it tax free.
You can indeed gift this money to your sister, but then as its more than £3000 you do have to consider any Inheritance tax - should you pass away within 7 years of making the gift, as it will then have to be considered along with your estate.
But again - unless you are UK resident AND your estate is worth more than £325,000 will this be of cost.
If more than 7 years lapse from the date o making the gift, then it drops from being considered for Inheritance tax.
But a gift, does not get paid back, so interest does not get charged.
So the final option is just lending her the money with no interest added, in which case, when its paid back, there is nothing to declare to HMRC, as you are not making any money from this arrangement.
Whichever way you decide to take this, make sure appropriate paperwork is drawn up, and it would be prudent to seek legal advise on this aspect.