I will endeavour to assist with your question.
First of all I would make two important points:
1. The information give is just that and is not intended to be legal advice (indeed due to Law Society rules it cannot be legal advice) so you should consult a solicitor if possible.
2. As this is a family matter, you should use all avenues to try and resolve this outside litigation and the courts (and the police potentially)
Starting with the criminal aspects there is obviously some potential for there to be criminal issues. The Theft Act states "A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it." Prima facie, it would appear that theft has occurred on your example.
There also appears to be a breach of Section 2 of the Fraud Act 2006:
The offence is entirely focused on the conduct of the defendant.
Those are the two most applicable based on what you have mentioned.
As to the civil side of things there are a few potential avenues to explore in respect of retrieving the money but all will involve suing, i.e. litigation and taking your brother to court.
First there is the 'tort' of fraud - meaning that the fraud in itself is an actionable civil offence that you can pursue at court.
There is also a possibility of a claim for negligence given your joint and several liabilities as executors but it could be tricky to prove. You need to show that your brother owed you and your other siblings a duty of care, that he breached that duty, that the breach caused a loss that can be quantified.
Both of these are 'torts' that can be pursued in the County Court or High Court
And finally you can simply file a claim asking the court to use its discretion to direct your brother repay the monies under 'equity'. Equity is a set of natural law principles which overlay our legal system and allows the courts and judges to use their discretion to apply equitable outcomes to grievances. To ask the court for an equitable outcome, you must appraoch the court with "clean hands" i.e. have acted reasonably in all of your dealings with your brother over this incident.
I believe that covers it, is there anything else you wish to know?
I think you have been extremely thorough. Thank you.