I’m very, very sorry that your family has so many complicated issues. But here’s the thing: You really don’t have a choice about whether or not your parents divorce. That’s their decision. It sounds like the two of them have been at odds for years. They are in a negative loop that is getting them nowhere. Your dad may be hard to deal with because he feels so disrespected and powerless. Your mom may be cheating because she gets little tenderness at home. They may both be emotionally exhausted and unable to do it any more. I give them credit for trying hard to manage for the family’s sake for years.
At 18, you are now one of the adults. I suggest you talk individually with your parents and let them know what you think you know. You may not have the truth. The situation may be more complicated than you think. You can certainly ask if there is anything you can do to help keep things stable for your siblings. Do talk to them about how they are planning to coparent if they do separate and what expectations, if any, they have for you. You deserve to be in on that conversation.
Encourage them to seek some counseling before they throw in the towel. But bear in mind that sometimes divorce actually makes things better. Freed from this painful negative loop, each of your parents might be able to become their best selves.
You might also personally benefit from having a counselor in your corner. You sound like a sensitive and thoughtful person. A counselor can give you some support and also help you look at alternatives for your life when your family is going through such a hard time.
I wish you well.