so i need some advice : my mother in law has been cheating on my father in law for 5 years. my…

so i need some advice : my mother in law has been cheating on my father in law for 5 years. my father in law works alot, pays bills, car insurance, pays for the house,etc. but he has caught her so many times talking to the same guy shes been seeing for so long but anyways my sister in law got in an argument with her last night while her dad was home and told her "you have no right to say anything because im not the married woman whose been cheating on my husband" and then my mother in law took off to el mirage and while she was gone my father in law told me if i knew about his wifes cheating and i told him, "yes i been knowing and i would tell your daughter because i dont wanna get in the middle of it. 3 weeks ago, she was taking me and my son(2) to his dentist appointment and she decided to pick up her FRIEND while we were in the car and i felt very uncomfortable seeing your wife with another man .. and not to mention a week ago my son was sick i told her i need to fake him in and she told me to wait until she gets back because she was gonna go out and eat with him and she was gone for 4 hours and i had to cancel his dr appointment because she came home very late" and my father in law told me that he wants me to grab the last 4 digits since i knew the first few numbers and give it to him and to write down the times that he calls her and if it is the guy he wants to file for divorcement but idk what i should do.. my sister is currently going through cps and if he kicks her mom out of the home they will take her baby away.. any advice on what i should do?