Hi nbc
First of all, is this a good idea?
You owe 25k on your credit card? Is there a good reason for building up this? Is it realistic to expect that you will be able to pay it off? Or put it another way, is there a risk that you won't be able to pay it off?
Most people who run up debts these size, have probably been unwise to some extent. If it's suddenly paid off, there is a very strong chance, that you will run it up again and you will end up owing €50k.
If you don't pay off a loan to your brother according to the agreed schedule, you could well fall out with your brother. This is not worth it for some small advantage.
People often believe that they can plan their finances well ahead but then their plans change. Your brother may well need the money sooner than the agreed repayment schedule.
If you do go ahead with it, he will be in receipt of interest, so he must pay income tax and prsi on it as with any other income, not just DIRT.
You must do a written agreement specifying the rate and the payment schedule and what happens if you don't meet the repayments. What if interest rates change? What happens if you die? What happens if he dies?
Borrowing €25,000 and paying €1,000 interest up front, is the same as borrowing €24,000.
For simplicity of calculation, it might be no harm to calculate the interest at the start of each year rather than every month or every day. A flat rate of 4% pushes up the APR a bit, but it's not that significant and you can adjust the flat rate if that is a concern.
If you are in a position to make a capital repayment, then you can adjust the interest charge mid -year.