First, realize that you and your parents have a mutual obligation to advise one another, especially when it comes to behavior that might be displeasing to Allah. How you go about advising a parent is quite straightforward in principle but perhaps very difficult and emotional in practice. In principle, you can advise your parents with respect and politeness, using non-accusatory language. In practice, you will find it challenging to try to advise your parents because they are supposed to be taking care of you, serving as role models for you, and, most importantly, advising you.
Second, try your best to talk to each parent separately and help them to feel comfortable talking to you about their problems. At a young age, you might think that there is not much you can do to help your parents, but, in reality, you can at least listen to them and help them to process some of their feelings of anger and frustration towards each other.
No matter how difficult it seems to you, your parents deserve to hear from you about the impact that their behavior has had on you and is having and will have on your younger siblings. If talking to them in person is challenging, then write down your thoughts and share them with your parents separately. Send them a letter or an e-mail. Let them know you care and that you are making du`aa’ that they change their behavior very soon!
Third, since they have been fighting, it seems, for so many years, the problems between them might be more complex than what help you can offer. We suggest strongly that you then recommend to each parent separately that they should both seek out a marital therapist to help them to repair and to rebuild their relationship.
Again, your suggestion could be rejected but it is worthwhile for you to at least help them to see therapy as a possible solution to their problems.
Finally, remember that ultimately you are a child to parents who have struggled hard to raise you and your siblings. No matter what, they deserve your respect and they also deserve to be advised about their actions. Make lots of du`aa’ and continue to seek the guidance of Allah as you advise your parents and help them to restore tranquility, love, and mercy in their marital relationship. And Allah knows best.