Do you think it's better for parents to stay together "for the sake of the kids" even though they're miserable?

I feel for you. I grew up with parents who fought all the time and it definitely affected me as a grown woman. Marriage was very scary to me because I had terrible role models growing up. I think you will find your way if you just remember the situation you are in is temporary. I know you love your parents but it is not your responsibility to make them happy. It is up to them. Just focus on educating yourself, looking after your sister, and gaining independence so one day you will have the life you dreamed of. I wish you the best!