How should I handle my father’s illegal relationships according to Islamic teachings?

Assalam Waleikum. I’m a non-muslim by birth but a true muslim in belief. From my childhood days I’m witnessing parents’ quarrel. Now I’m 26 yrs old and can well understand the reasons for that. I’ve discovered that from early days my father has been getting involved in multiple illegal relationships (sexual relation as well) sometimes with married women, divorsee, and presently with an unmarried girl younger than me. A few months back I accidentally found them both in a room in ‘uncomfortable position’ but I didn’t protest openly. My father also remains in contact with that girl via cell phone almost everytime which is very irritating. These are not unknown to my mother. My parents divorsed and re-married again about 7 years ago. Both of my parents stay in separate places (also because of jobs) but have no actual contact (only for show). In such a situation, I’m helpless and worried of my mother’s mental condition. I cannot understand what to do in such a situation as per Islamic way of life. Whether to protest strongly or to leave everything on Allah. Please guide me. N.B.- Because am a non-muslim, I cannot understand the Islamic terms. Pls give the English meanings as well. Thank you. Assalam Waleikum.