Dad's extra marital !! Help?

Well. I just discovered that my dad s having an xtra marital. MY mom's v stressed out with this moreover i n my brother are leaving home for studies. My mom would be absolutely alone out here. My Dad is not the kind who would stop caring for us or something but this s somethin whch is getting impossible for me to handle. I am highly depressed. I dont even talk to my dad these days. I feel ashamed of him . every time i go near him.. i start getting angry n irritated. My mom keeps spying in him but doesnt wanna do anything. SHe keeps explaining to him at night. She keeps asking him the truth but he refuses all the time. I myself have seen messages of this OTHER lady on my dad's phone which has put me in a state of shock.. I feel like banging this Other Womans head into the wall.. OH God!! PLzz help people.. what should be done?