The fully comp bit relates to the cover you have on your own vehicle. You must look at your insurance certificate to see whether it allows you to drive other vehicles not hired to you or owned by you. If it does then go ahead, but beware you have only basic third party cover when using that car. ie. if you stuff it you have to pay for the damage to your brother's car. Fewer companies are allowing this cover nowadays as it has been abused in the past.