Welcome, Tamsin. You'll find many, many people here who can relate to all you've experienced.
Like you, I'm the family scapegoat. And like you, there are some who can't understand the problem because my parents and siblings are soooo niiiiiice. My children, sadly, are among those. The ugliness all happens in private--in phone calls and e-mails--where nobody sees.
I've found it very healing to be where people understand and know and reassure me, that I'm not crazy. Because it sounds crazy to those who haven't seen it first hand.