I’m so sorry your parents aren’t shielding their children from their difficulties. You should not have to live with this. Your parents’ fight is with each other, not with you. Only they can solve whatever is troubling them.
Since you are only 14, there are only a few things you can do. You tried to tell them that it is distressing to you. Apparently, whatever is going on between them is so big that it is even bigger than their concern for you. That means that it’s very, very big.
I can only suggest that you and your sister not add your noise to their noise. When they fight, leave the situation or leave the house.
Help your sister understand that picking on her is a way that your folks are distracting themselves from their own fight. It probably doesn’t have anything to do with her.
The best thing your sister can do is find a way to gracefully refuse to be part of an argument. Instead of arguing, she can say something like “I’ll think about what you said” or “You may have a point”. That’s always true. If she can say it genuinely, and without anger, it will help her extricate herself.
If you have a grandparent or other relatives you trust, you could confide in them and ask them to talk with your parents about how to settle their differences. Sometimes adults are more willing to listen to other adults than they are to kids — even their own kids.
Meanwhile, you and your twin can spend more time with each other, with friends, relatives or at school activities so you don’t have to be constantly stressed by this stressful situation.
I wish you well,