The list of disqualified persons in the code covers you as the account owner, your spouse, your lineal ascendants and descendants (parents, grandparents, children, grandchildren) and their spouses, anyone acting as a fiduciary or service provider to the plan, and entities that those people own or control at the 50 percent level. Siblings, aunts, uncles and cousins are not named. So a loan from your IRA to a brother is not automatically a prohibited transaction, though whether a specific arrangement crosses the line depends on the facts and on how the statute applies to them, and that's a question for a tax attorney rather than a forum.
The thing that turns a permitted sibling loan into a problem is indirect benefit. If you get something out of the deal personally, a fee, a share of the flip profit, use of the house, forgiveness of a debt he owes you, the transaction can be attacked even though he isn't on the list. Also check the ownership of whatever entity is borrowing. If his LLC has you or your spouse as a member at 50 percent or more, that entity is disqualified even if he manages it.
Mechanically the loan has to be the IRA's loan and not yours. The note names the custodian for the benefit of your IRA. The money wires from the IRA. Payments go back to the IRA and never into your checking account, not even for a day. Terms should look like what an unrelated lender would write, secured by a recorded mortgage or deed of trust (which instrument and how it's recorded varies by state).
The reason people are careful here is the penalty. A prohibited transaction can be treated as distributing the entire account as of the first day of that year, so the whole balance becomes taxable at once.
