Giving a loan to somebody is a zero-sum game wealth tax-wise. In your case:
- Your wealth decreases by 15k as you have this much less in your bank account
- At the same time you have to declare the loan to the tax office, and guess how much it is worth - right, 15k. Consequently, your wealth increases by 15k right to the level where it was before.
One could of course “forget” to declare the loan, which would even work with small sums. In case anything goes wrong with the repayment of the loan and you need to go to court it could end very badly though.
Any interest received should be declared and will be taxed as income.
Edit: apart from tax implications, please consider worst case scenarios and make sure to draft a proper contract for the loan. What if your relative can’t (or won’t) repay the loan? What will you do in this case? Things like this can ruin family relationships.