Aly, my depression began in my early teenage years, due in part to abuse from my mother. It took me decades to get it properly diagnosed and treated, and since then I've taken medication. The medication helps level the playing field, but I still have to work at staying positive every day.
One thing in your original post jumped out at me. You said that a major trigger for you was hearing about how awful your sisters were. For me, I had to distance myself from my mother in order to survive. I didn't completely cut off communication, but phone calls were short and impersonal and visits were rare. Maybe you could consider distancing yourself from your sisters, at least for awhile. Taking care of yourself is the priority.