Looking to feel less alone

I am the carer for my 14 year old niece, my younger sister's daughter. My sister is currently doing DBT for the first time despite having been diagnosed with BPD over 4 years ago. The story with my sister is a tough one to talk about. I do not have a good relationship with her most recently because I am caring for her daughter.
My niece is suffering badly with depression, is chronically suicidal, self harms constantly and has PTSD from the myriad of traumas she has suffered in her life.

she is a gorgeous loving kid and I love her as though she were my own child. She has lived with me for three years now (prior to that she spent 7 years living with my parents but had to come to

me for high school as there wasn't a high school near my parents home.)

We are truly struggling right now. She has come out of hospital after a 3 week stay in an adolescent mental health unit, and she is very negative about the possibility that she can ever get better.
every night is a Mexican stand-off as I wait for her to fall asleep so that I can sleep. My husband is fabulous but it is me that has to do these nights as he is the major income earner in our house and somebody has to pay the bills.

i am furious with my sister for her inability to see how much her daughter needs her,(the only contact they have is when my sister calls at random intervals and usually tries to manipulate my niece in some way). and although I know she has illness herself I do not know how to have compassion for her when I know that she is the biggest cause for my niece's distress and a large portion of the trauma she has suffered.

i am hoping to hear from some people who are struggling in the same ways I am. I don't feel very strong and I know that I will need every bit of strength I can muster to survive this.