No bank (including the Post Office) would consider reimbursing the money unless you could first show that you'd reported the matter to the police.
Even then there's no evidence (as far as the Post Office is concerned) to show that the withdrawals were fraudulent. As far as they know, your stepfather could have given his PIN to your brother and invited him to withdraw the funds. Indeed, they will always assume that someone making a withdrawal is the account holder, or a person authorised by them to make such a withdrawal, unless there's clear evidence to the contrary.
As an example, a colleague of mine had money taken from his account without his knowledge on several occasions. He eventually found out that his sister's boyfriend had been borrowing his bank card from his wallet while he was asleep. However he still couldn't get the money refunded as the bank (correctly) ruled that he must have made his PIN known to the thief, either by writing it down or by letting him see it when he entered it into a cash machine.
So the chances of you getting the money refunded from the Post Office are almost certainly NIL.
You can either report the theft to the police and hope that the court, as well as punishing him for the offence, will make an order for him to repay the money to you or you can take civil action against your brother.
To proceed with civil action you would first need to send him a formal demand for repayment of the money. That demand must:
a) show your name and address ;
b) show his name and address ;
c) be dated ;
d) clearly indicate that it is a demand for payment ;
e) clearly show the amount demanded ;
f) clearly state the reason for the demand ;
g) indicate that if payment is not received within a defined period (of, perhaps, 28 days) you will commence legal action in pursuance of the debt ; and (obviously)
h) be signed by you.
It would also be best to send the letter by recorded delivery (even if you see him daily), so that you can prove that he received the demand.
Then, if he fails to pay within the designated period, you can use the online small claims procedure to seek a court order against him:
https://www.moneyclaim.gov.uk/web/mcol/welcome