Saw the batsignal.
Okay first, escalate to a rep that understands inherited IRAs ASAP. If you're just calling in to the 800 number and your dad didn't have an assigned rep, then make sure to get to someone on the line - and their name and a dedicated number to contact them directly - so you have a point person to get this stuff taken care of ASAP for you and the other beneficiaries before the end of the year. They should not be bouncing you around to whomever answers the call at this point since it is vital to get the RMD thing figured out in less than 2 months.
You need to specifically tell them "I'm worried about this account and really need to get a dedicated representative that is experienced and knowledgeable with dealing with inherited accounts. Please transfer me to such a rep that will be able to be the point person for the heirs of this account so we can make sure there are no issues or errors with the transfer and required distributions."
So get the person to confirm that the RMD is taken or not, and if it needs to be done, then have them do so. They can cut a check to the estate/beneficiaries (clarify who should receive at this point), or you can have the money shifted to a taxable account (if one exists), or ask them for any other options.
I personally did not have to deal with this part as my father had fortunately taken his RMD for the year already (which was great because he passed in December so it was pretty short notice to deal with all that and his actual death).
If you and other siblings were named beneficiaries on the account then wrapping it up will have nothing to do with any will/executor. This is actually the best/streamlined way to pass an account over the heirs. Yes, you each will need to take care of your own accounts.
This is how the rep I had explained the process:
First, we each had to create our own accounts, inherited IRA/BDA with our father's name specified in the account name (they will explain this, but it is vital that the account type is named/correct as far as it being an inherited IRA).
Next, whomever has a copy of the certified death certificate will need to send that in to them (mailing is fine - make sure to send with tracking to ensure delivery). Along with the death certificate, they will need to send a letter stating that they are one of the named beneficiaries for the account of DECEASED NAME (include his existing account number), and you are asking them to split their portion of the inherited IRA into your newly created account inherited IRA (provide account number of new iIRA account) for yourself and NAMED SIBLINGS/CONTACT INFO. I can't remember at that time if any funds held in the primary account had to be sold off before transfer, but we asked that it all transfer over "in kind" where possible and sell off anything that was a proprietary fund (if you dad had their professional management group, there are some funds you can't hold without also enrolling in professional management/advisory accounts). The rep should advise you on exact wording, but it was not complicated at all.
My sibling had access to the death certificates, so they sent it in and I only had to mail in my letter of instruction just referenced that "death certificate was provided by SIBLING NAME/CONTACT INFO, mailed on or around x date" and the rep easily located it and linked up so we didn't have to send multiple certificates.
Finally, make sure that everyone sets up automatic distributions (the RMDs) for their accounts so there is not surprises a year or 10 down the road when they suddenly realized they should have been taking them all this time. You can do this part online pretty easily too, but if you or any sibling is uncomfortable, again utilize the point person rep to have them walk you through it on the phone. They should be able to do so (mine did).
You don't have to do this right now, but one thing to also do in the next 6-ish months: you all get stepped up basis on any holdings in your father's accounts. So even if he had major capital gains on his funds in his portfolio which would mean lots of $$ owed tax-wise depending on the taxable brackets he sold off any funds while still alive, with his death the cost basis is reset to zero. So you can (and probably should) sell off everything as soon as you all figure out what you yourselves would like to hold in your portfolios (unless he had exactly the sort of mix you're happy with).
Cost basis isn't going to go nuts in~6 months while y'all mourn and then do a bit of research on the types of investments that works for you and make your moves once you're through the worst of this. But get your dad's RMD figured out, then get the accounts separated, distributed and RMDs in place and take a breather if you need it.
And finally, I am so sorry for your loss. It is so overwhelming trying to deal with the intricacies of money and investments at the same time as dealing with losing a loved one and also worry over all the things one has to deal with when there are multiple family members trying to deal with this loss as well. Take care of yourself, and do seek help at any time things seem difficult and never feel bad about needing to take a bit of time on this.
ETA: fixed deceased RMD distribution info as I wasn't sure who should get (so basically reminder to go over details with authority on subject to confirm actions to be taken)