I think the problem is there is a general misunderstanding around many mental health conditions - I couldn't agree more, JJXM. Mind & Rethink do good work here, and I always spread their messages as widely as I can :)
For my next comments, the highly sensitive might prefer to look away 
McG, you said of your relative: "A lot of her behaviour made no sense. ... I found ways to deal with her. I realised she would never react in a normal way"
This is hugely important, imo. The old-fashioned word for some Personality Disorders is "mad"
Subjects with such impressive lack of self-insight will never seek diagnosis and are, for the most part, unlikely to be sectioned. When I'm talking to the partner of someone like this who will, along with their children, have been torturing themselves with agonised soul-searching "Why? What have I done wrong? How can I make him see?", then I am more than happy to suggest there's actually no external logic to the behaviour. If I can point the unhappy poster to descriptions of disordered behaviour that match her lived experience, so much the better.
I'll go a step further - privately - when the abnormal partner seems to be "just an abusive arse". Being an abusive arse is not a successful life strategy in peacetime. I figure, therefore, that an abusive arse who continues with their maladaptive behaviour - despite predictably adverse consequences - has a personality disorder (no capitals.) Whether or not they'd be diagnosed with a mental disorder by the clinician they will refuse to see, they display inflexible patterns of thinking, feeling and behaving which cause problems for them and people around them.
If I happen to know of a diagnostic matching the described behaviours, I will point my reader towards it. Not because I think I'm a telepathic internet psychiatrist. Because it will help the poster.