Is it a Roth IRA or a traditional IRA? You mentioned the 5 year rule, which is relevant for Roth IRAs.
An Inherited Roth IRA is not taxable, when distributed.
A Traditional IRA id fully taxable, unless you have records of a tax basis (non deductible contributions made by your parent, in the past). Any tax basis will usually show on form 8606 of your parent's last tax return. Tax basis is fairly rare and even when they exist are usually a small fraction of the total.
