Working through my thoughts again...

Mum just came over, she seemed happyish. But it doesn't matter what subject I bring up it turns negative and stressful and makes her upset or angry. So now I feel bad.
She says she never has any money and she's struggling but I try to offer sugetions and she gets mad. Saying I treat her like an idiot. I'm not trying to upset her, I just want to help. So she won't be so unhappy. And niether will I.
No body seems to care how I feel. Wether Im sick all the time, wether I get so down I want to throw myself off the roof jut to make it all stop.
My mum and sister argue non stop. I have to listen to it all. Because they both ring me and tell me their side of it or to bitch. And I can see they both have points etc. It just stresses me out beyond belief because there isn't anything I can do to stop either of them feeling stressed and unhappy. And it makes me feel even more stressed and unhappy then I already feel.
I'm not a mentally healthy person myself and they expect me to carry all of it. But I can't say anything. And I feel like a horrible daughter and a horrible big sister because I can't make all the horriblness we have all been through go away.
I don't know wether I should just listen and try not to think about it too much. Just let them vent. But then I feel guilty.
What I want is to just be left alone. I'm surrounded by so much negativity that it's killing me. My mum has bipolar and my sister is losing her mind. And I just want to say well I've done all I can, you don't want my help in the way I can give it and it's all I have left to give. But I feel like a terrible person.
Should I try to protect my own fragile health or sacrifce it for my family.
The problem with that is Im engaged. I have a house I have a job. I need to be able to put energy into those things as well. My poor partner. He never ever has gotten what he should of from me. He's so understanding. I want to give him what he deserves, instead of comming after all the stress my family puts on me.
I just don't know what is selfish and what is self protection anymore.
I fell like Im selfish if I don't fix or try to fix my mums problems because if I said no I just can't do this anymore she'd make me fee like the worse person ever and then not talk to me ever again like its me that is being unreasonable. I love her so much...
But I want a life.