Talk to your mom and explain that it puts you in a very difficult position. Her carelessness with the phone is something that will certainly cause an issue with your dad, but you cannot try to manage your mother. You can only explain how her behavior has put you in the middle. The important thing to do now is let her know that if her 17-year-old daughter can see this stuff, so can her husband.
I would also be making plans to get out of the home as soon as you can and begin college. Your mom and dad are going to have to deal with their issues — and you are going to have to begin the process of figuring out what you need to do to help yourself. If you have older siblings, they may be the ones you can talk to. If you don’t have siblings, you may want to use our online forums to chat with others to get some support as you cope with this.