Although the bank account may be in joint names, that only equates to the legal title of the money.
The money in the account belongs to whoever "owned it" when it went into the account.
If for example all the money was your mother is from her pensions and benefits et cetera then it is part of her estate.
If however some of the money was yours, that money belongs to you.
This is where the situation becomes difficult.
If all the money was your mother's then that money belongs to the estate and should be passed over to your brother as executor of the estate.
If however you had money in the account as well which it seems that you did then becomes what is known as "mixed money" because it is not possible to identify which money is yours and which money belonged to your mother.
In that case, it does not get passed over and if you do not willingly volunteer it, he will have to take you to court and the court will have to decide whose money this is.
It is not unreasonable for you to let him have bank statements going back a couple of years.
It is completely unreasonable of him to want details of all the expenditure going back all that time unless there are any abnormal items which would flag up as being unusual.
It's also not unreasonable for him to query any amounts of money which may have been payable to you or in respect of things for you.
Can I clarify anything for you?