Yes you can. The interest not charged would be seen as a gift for tax purposes, but you can gift someone up to €3k a year without any tax consequences, that would cover the interest.
Strictly speaking you do not need any documentation, but it would be wise to draw up a loan agreement , for two reasons. To confirm for tax purposes that the €15k is a loan and not a gift, and to ensure that both you and your sibling have the same understanding of the terms of the loan.
Nothing complicated needed, I don't see why you would need a solicitor, just write down what you intend for the loan, two copies two signatures on each.