Sorry to hear about your loss.
Assuming that you're referring to Dad's monthly pension, it's usual for a spouse to receive a reduced amount of a company pension; Mum got 50% of Dad's. Regardless of what's written in the will, this is new money and belongs to your Mum so it needs to be paid to her account.
If there are other pensions, these should show on the bank statements so these need checking.
If you take your Mum to the bank with Dad's death cert, the joint accounts will be transferred into her sole name. It's now her money not yours or your brothers.
If there's property involved, you need to find out who's name it's registered in and what sort of tenancy there is. If it's joint tenants, Dad's name is removed and the property becomes your Mum's, tenants in common would mean that your Mum now owns half and the remaining portion become part of his assets i.e. you and your brother own 25% each.
Please be aware, that the will could be challenged if your Dad has failed to provide for your Mum so you may want to get a solicitor involved.
@nitram you're the expert so please feel free to correct me.