We need to know more information to be able to offer appropriate advice.
I assume your brother's Oyster card has a discount or free travel applied to it?
Where did this happen?
What was said to you by the person who caught you?
How old are you? Are you entitled to your own discounted Oyster card?
How many times have you used this card?
Transport for London will write to you saing that they have received a report of fare evasion and invite you to provide an explanation or comment. You can also tell them about any mitigating factors you might want them to consider.
TfL normally take a dim view of this sort of thing and it is likely you will be prosecuted in the Magistrates Court for an offence under the Transport for London Bylaws. In certain limited circumstances TfL has been known to offer a final warning.
Further details can be found on page 10 of TfL's Recenue Enforcement and Prosecutions Policy: