Maybe I'm missing something here.
But, as soon as the death certificate is signed, all accounts & assets of the deceased are frozen. The sister wouldn't be able to get into any accounts or even the safe deposit box, even if she is co-named on the account, unless she was listed as the Administrator of the estate. And it sounds like she petitioned to be and the brother & OP
allowed it as the OP said:
It also doesn't sound like the sister is trying to cheat or swindle the siblings out of money or assets or the sister, as controlling as she is, also wouldn't have said:
The sister would have planned a trip by herself and pocketed any assets that aren't listed. But, she purposely ASKED the siblings to go with her, so they could open and look at all the accounts TOGETHER.
She also said upfront:
Again, if she was planning on cheating, lying, etc. she would have hid the fact that she is planning on taking out the money from the inheritance until AFTER. As controlling as she is, she IS telling the OP what she is planning on doing.
Some of you are saying "Hire a lawyer." For what? How much is a third of the cheap hotel bill? Another $30? I'd also say, "Fine take it out of the inheritance."
To hire an estate lawyer, the retainer will be between $200 - $500. To REMOVE an already appointed Administrator will be several more hundreds of dollars as it has to go to court. THEN, who is going to be the Administrator if the sister is removed? Doesn't sound like the OP or the brother want it. To hire a Public Administrator to oversee the case, depending on the amount of assets, may cost several hundred more dollars. It doesn't sound like there is a lot of an estate if the sister is getting a cheap hotel room for the 3 of them to share. OR she would have charged a better one to the estate already.
All that for a third of a hotel bill.
The sister being petty & controlling doesn't necessarily equal lying & cheating.