Thank you for your query.
The witness statement (COP24) allows your parents, as the people applying for the Order, to give more information about your brother's situation to the Court of Protection. A witness statement can be completed by your parents and by other people who are able to give relevant information. It also allows your parents to state why they feel that appointing them as the Deputies is the right decision for your brother.
COP3 is the Assessment of Capacity and the 'further information' section allows them to give any relevant information about your brother's capacity.
COP4 is the Deputy's Declaration and the 'personal statement' section is for the proposed Deputies to set out why they wish to be the Deputies.
The information provided on the forms needs to be specific to your brother's situation and needs to demonstrate to the Court of Protection why a Deputy Order is appropriate in the circumstances.
I hope that is helpful but please let me know if you have any further queries.