I believe if your sibling is old enough, they actually have ownership, regardless of whether they've done anything with the account or even know it exists!
The UTMA is a taxable account already. Your sibling should have been reporting any dividends and capital gains on their tax return since you started the account. You'll have to ask Vanguard whether you can transfer directly to an IRA, or if you have to move the money to an intermediate location first. Regardless, your sibling would have to pay capital gains tax when selling the funds to move to an IRA.