Condolences on your loss.
That's on the slow side from what I've read.
It's my understanding, though, that usually inherited accounts are opened at the broker (MS in your case), then the assets are transferred into those inherited accounts. Then if you want to transfer your inherited account to another broker you can do so at that point.
Were I in your shoes, I would first verify that they have everything they need (they'll need your SSN, name, birthdate, KYC stuff probably, and same for your sibling). After that, if they don't have it done in a week, I'd start asking why and escalating. I've heard that informing them you're going to contact the federal CFPB might expedite things. There also might be an office of the MS president which receives escalations.
You should ask them to make sure the brokerage account has the basis stepped up to your Mom's date of death. You'll also need to know if your Mom took her year of death RMD if applicable.
