AM alarm + only one fob = problems

Hello, my family got a 98 musting a while back and it came with an aftermarket alarm system installed. My sister is the only one with a fob since she usually is the one to drive it, but when she is not around we cannot use the car because the alarm is active and there is no other way to turn it off. Since trying to make my sister remember to not arm the car is harder than replacing an engine, I wondered where I would start in trying to get another fob for the car.

Where would I look to find the alarm system to get the info about it? I'm pretty sure my parents wouldnt know where to look up the previous owner at this point and time...

thanks in advance