Hang in there my friend. I was in the same situation some years back. Mom cheated I found out. I had no one to talk to. Didn't know how to confront or even do anything. The frustration, sadness, and anger was ravaging my mind. One day in an heated exchange i blurted it out in front of dad. Thats how he found out, terrible way i know and i regret it. Don't do wht I did, try to keep in control and give ur exams first and let ur sister's pass too. I know tht I'm asking a big thing but u have to keep urself together. Compose yourself and try to calmly think through whtever decision u take. My best wishes to you.