The whole point of any IRA is that the contributions are tax-deductible and the investments grow tax-free, so there is no basis and the entire amount is taxable as regular income. (Even though, with a regular broker account, you are only taxed on your gains and usually at a lower rate, IRAs still have a strong financial advantage in the long run.)
It is possible, but uncommon, for someone to contribute non-deductible funds to an IRA and have a partial basis that is not taxed on withdrawal. But to prove this to the IRS, you would have to have copies of form 8606 from the deceased's tax returns. Without that knowledge, you will assume that you have zero basis and the entire withdrawal is taxable.
