Things have gone from bad to worse. What do I need to do?

So it all started in January 2015. I was home for a few more days of vacation before I had to go back to school and I found pictures of my dad with another woman on his iPad while I was playing a game. I got curious so I found out more about this mystery woman. It turned out that my dad had been talking to her for a few months before and had met her while on a business trip in a different country. After finding out about it I was devastated and wanted to tell my mom right away. The only thing that kept me from doing so was my little sister who is 9 yrs old. I don't want her to grow up without a mom or dad so I've kept quiet.
Going forward to February was a time that things started to go south with my older sister (22 yrs old I'm 20 yrs old) They involve her being kicked out after a number of things that went wrong. Moving forward again to July 2015. Everything that has happened before this I believe has been building up tension in my parents relationship. A few days ago my mom shared a text with me that she got from my dad. In it he said that he "wouldn't be around for much longer anyway…"
With him saying that to her it makes me want to tell her about my dad's cheating just so she can be ready for when he does leave. My mom already has some suspicions and I think I should tell her I just don't know how to bring it up. They have been married for 23 years. It's just really hard to imagine them apart.

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