You say that your brother received a speeding fine. I assume you mean that he has received the paperwork for the offence as he is the registered keeper of the car, and has been asked to provide details of the driver.
If the matter is to be dealt with by way of a fixed penalty (in which case the offer of a fixed penalty should have been provided along with the request for the driver�s details) then the penalty is the same regardless of the status or experience of the driver (�60 and three points).
However, if the matter is referred to the magistrates� court (and this usually happens when the speed is so excessive as to be outside the limits for a fixed penalty) the punishment will be harsher. They can impose a higher fine, court costs, up to six points or a disqualification. Their sentence will depend mainly upon the speed recorded, but the magistrates� may also increase it because of your sister-in-law�s inexperience.