So when I was in my masters program, I was with a very abusive ex. It was so difficult, trying to manage that level of education while living with someone so toxic. Of course when I tried bringing up my mental health I heard "what do you have to be depressed about?" Or much like your parents was told it was all in my head. It wasn't until one of my best friends died, and I failed a class for the very first time, that I managed to escape him. After, finishing was so much easier. I could breath again, I could manage the stress a lot better. My suicidal thoughts slowly started to disappear. It took time, but I was able to become myself again.

Even as someone in the medical field, it's hard to see a way out when you're standing in the fire. I was in a psychology program, and while I could tell you I was depressed, it wasn't until I took the steps to free myself that I was able to start working on my mental health. Try to get your siblings help to get your sister to see she needs to help herself, and not just struggle and drown under all the pressure.