What should I do?? :/?

Okay, I have my brother's car because he went on a trip to Europe(we don't live together), so he gave me the keys to it, first question is.. Am I allowed to drive it?? because of insurance reasons, etc... I have a license and all that, but my mom is saying "no!". Also, part 2 of this is there is this girl I liked/talked on again off again(way out of my league btw...), I told her I have my bro's car for the week, so she asked me to pick her up(she lives 40 mins. away), and we'll go see movie or eat somewhere(hopefully get lucky haha), HOWEVER... my mom is saying like "no, no, no, you're not going anywhere, you're not allowed to drive insurance, blah, blah, you're inexperienced blah!" which is getting me pissed, she won't be home at the time when I arranged the date with the girl.(around 6 pm, she leaves work at 3), should I just take the car and go? or should I call the date off? how can I convince my mother to let me have a life, im 18 and am i even allowed to drive my bro's car? thanks for feedback!. :)