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i borrowed my brother's car and put in 91 fuel instead of 95 fuel, will this affect how the car runs? Is he able to use up the 91 fuel 'til empty and then refill with 95 or does he have to drain his petrol tank of 91 and then refill with 95?

Firstly, you should always use the manufacturer recommended minimum octane rating fuel (or higher)in your car. If you use a lower octane it could cause severe engine knocking - caused by the fuel pre-igniting. Prolonged heavy knocking could lead to engine damage. Most new cars have engine computers that will detect knocking and retard the timing to try reduce the knocking. If the knocking is noticeable and performance is noticeably compromised then you might consider draining the fuel and filling with the correct type. If you cannot notice a big difference then you might be ok in this instance.