To my sister, will this override the beneficiary form I signed resulting in the trust holders wishes that my sister receives the funds?
You can disclaim an interest in an IRA and it will go the next listed beneficiary, i.e. the contingent beneficiary. If there is no contingent beneficiary, it goes to the Decedent's estate unless the IRA has default beneficiary provisions which may give it to someone other than your sister, i.e. a spouse.
If you have already claimed the IRA or more than 90 days have past, it may be too late to disclaim. If you wanted to give it away after claiming it, you would still be responsible for the tax on the income.
You should talk to an attorney because there can be substantial tax implications to a botched disclaimer.
This answer is intended to provide legal information, not legal advice. Legal advice should be provided by licensed attorney only after full disclosure of all facts. If you desire a no-obligation consultation to obtain legal advice, please contact me at 586-268-4463.
As Mr. Vadar points out, there could be significant tax consequences. If there are no contingent beneficiaries your disclaimer, if timely, will cause a probate and adverse tax consequences from accelerating the time for distributions.
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Estate Planning Attorney
As my colleagues above noted be mindful of the period for disclaiming the funds may end up going through probate and incurring some tax liability.
Estate Planning Attorney
Best way to handle is to have Mom simply amend her Designation Form for the IRA account naming your sister as 100% beneficiary and you as contingent in case she were to predecease you. The custodian of the IRA will have a standard Beneficiary Designation Form for your Mother to sign directing this to be her wishes. This avoids any probate issues and disclaimers and no need for lawyers to fix this issue.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.