The IRA custodian might try to help determine the RMDs, but they don't always get it right and they are not required to do so for inherited IRAs. Ultimately it's the responsibility of the trustee of the trust to request the necessary distributions, so I would rely on the advice of the trust attorney or another knowledgeable tax professional like a CPA. The distributions would only be based on the age of your father if your father was the same age as or younger than the girlfriend and the RMDs would otherwise be based on the age of the girlfriend.
I don't know if the RMD calculation would revert to using your age if the girlfriend beneficiary dies.
It's not entirely certain that the SECURE Act changes weren't taken into account when preparing the trust document since age of the beneficiaries is a factor in determining RMD both pre-SECURE Act and post-SECURE Act. The preliminary regulations weren't published in the Federal Register until February 24, 2022 and the final regulations didn't become effective until September 17, 2024, so things were still somewhat uncertain when the written.
From the information you've provided it's not clear that your sister and the girlfriend meet the definition of special-needs beneficiaries, but if they do, but it still doesn't seem that the requirements are met to be able to disregard the age of the girlfriend (assuming that the girlfriend is considered to be a beneficiary under the circumstances) in determining RMDs paid to the trust unless the present trust document specifies that sub-trusts are to be created.
"my father’s lawyer should have made my sister-in-law and me 25% beneficiaries on my father’s IRA and the Trust a 50% Beneficiary for my sister and my father’s Trust."
I agree that that would have made more sense if the intent was to simply divide in equal shares, but the result would be different if the intent was to allow the other trust beneficiaries to receive the income when one or more of the trust beneficiaries dies.
Does the present trust document specify that sub-trusts are to be established? If not, I don't understand how sub-trusts would be permitted to be established and, even if permitted to be established, it seems that they wouldn't be established under the terms of the present trust to allow those trust beneficiaries to be disregarded in determining the RMD for your share.
