Hi Blissfulblues
Your brother should be able to request a three way telephone call so you could be on his assessment as well.
Do look at the up-to-date information on telephone assessments in our ESA/Guide. The information is on page 117 onwards.
www.benefitsandwork.co.uk/help-for-claimants/esa1 You can ask for the phonecall to be recorded might it not actually happen. If your brother has the means he could make a covert recording.
BIS