It's funny - my sister was more the 'coper' role, and I was labelled 'the weakling'. Picked on and laughed at by my father. In later years, made to feel that I'm over emotional, unstable, needy. I ended up on anti-depressants but managed to pull myself out of it.
I was in and out of short relationships, unable to find someone to settle down with, generally unhappy. My sister got married, had 2 lovely DCs, nice home, and was 'the settled one'. She's always been the one to deal with family logistics, doesn't argue with DF as badly as I do etc.
Then I found someone wonderful. We're strong and happy together. There is some sadness - we're ttc and there are complications - but other than that our life is wonderful.
Then DSis monumentally fucks her life up - has an affair, leaves DH, OM dumps her, she's in the middle of a breakdown. Single mother in a council house, massive debt, children emotionally neglected. My parents have behaved in a bizarre way (I had many threads about this), they have minimized and sometimes lied about my sister's behaviour. They blamed everyone else, and fell out with me for not doing the same. They are bitter and in denial.
I have fought a long, tough battle to get through this. My relationship with them has been permanently damaged. I expected my DM to stand with me and give my sis some home truths, condemn her behaviour etc... but she went the opposite way and stood with my sister and condemned me. They actively encouraged her affair, and criticized me for now doing the same.
I admit I am critical of my sister, and my DPs take it very personally. I know it's because they're so worried and hurt, and they just cannot admit she's to blame. They see their role as defending her, and keeping up the facade of a unified family unit at all costs and the strain is huge.
So I'm the scapegoat. I'm often told "It's ok for you, you have your DP / plenty of money / lots of holidays etc". But I have all of those things because I've fought for them, and worked hard, and saved and been careful... and my sis would have that too if she had behaved more responsibly. I think they know that, deep down, but will never admit it.