Yes it is dangerous to drive with a crack in your windscreen as it can develop into a larger problem as time goes by, especially if the windscreen recieves an impact like a flying rock or gravel.
Have your sister check her insurance policy as most include one free windscreen replacement per year. You can purchase stickers for small cracks from car care stores and some windscreens can be repaired without the need for replacement. Take the car to a windscreen repair centre and have them assess the damage.
Another good idea is to pop a copy of Galmatic - the Car Care Handbook for Girls in your sisters glovebox!Happy and safe cruisin'
www.galmatic.com
Georgie
Galmatic - the Car Care Garage for girls. Car care products and information to keep you cruisin' in style
http://galmatic.com