pko outlined one method of handling this situation.
However, there is another way to proceed if more time is desired to complete the disclaimer, but subject to the 9 month deadline. Per IRS RR 2005-36 (see link), the RMD can be taken by the beneficiary without eliminating her option to disclaim. This RMD would be taxable to the beneficiary who receives the funds. For example, if your father passed in late May, your mother would have until late Feb, 2011 to complete the disclaimer per Sec 2518. But the RR allows her to first take his 2010 RMD prior to year end and then to complete the disclaimer early next year. So this is a timing issue that can avoid having to request a waiver of the 50% penalty, even though the IRS is almost certain to approve the waiver.
But this can also be done as pko indicated, subject to the primary deadline of 9 months for the disclaimer. In that case, the disclaimer is done first, then the contingent beneficiaries satisfy the RMD, which is again taxable to the beneficiary that receives it in full or in part. In no event is the RMD reported on the estate income tax return of your father.
http://easysite.commonwealth.com/EasySi ... %20IRA.pdf