if you are are in recipt of income support you would be eligible for the carer premium which equates to £29.50 per week. However, you need to take into consideration that if you claim this it may affect your brothers benefits - for example if he currently receives a Severe Disability premium, he may no longer be entitled to this and overall you could end up worse off.
Best thing to do is seek some advice before claiming. You need a benefits check, where are you located? Most councils should have this type of welfare rights service, if you contact them they should be able to help you. Council contacts can be found here, along with a lot of additional information on benefit entitlements: http://www.direct.gov.uk/en/Dl1/Directories/index.htm