Rooty - to explain a bit about why I feel this way - distancing quite literally saved my sanity when I was trying to deal with the hurt of a similar problem with unbalanced family dynamics. I don't think that it does always end with separation - it can be a key to retaining some kind of dynamic.
Anyway, here's my story. My sister is an arch manipulator. She is in her mid 30s and has always lived with my parents despite having a very committed long term relationship. She has isolated them from the outside world almost completely: they rarely leave the house except to go to the supermarket, they have no friends, and they are unhealthily focused on the comings and goings of their neighbours. My sister, by contrast, has a very good, well-paid job. While she is at work every day, my parents run around and do her washing, cook her meals, and generally act like live-in servants.
So that's the practical arrangement. Now for the emotional side of things. Over the years, my sister has built a toxic bubble with them, in which reality is warped into a strange mirror-image of itself. And that reality has been carefully, painstakingly constructed with two objectives: to enrich herself and to exclude me. It started out with little lies and twistings of the truth, and its ended with a complete break between their reality and mine. Whatever I say or do, however innocent, is twisted to give it the most awful, negative interpretation possible. (Even the fact that I like Christmas trees and put one up in my own home in early December, against a family tradition of doing it on the 24th. I kid you not, this has been taken as evidence of utter malice on my part!).
I've tried to confront this in every way you can think of. In the early years, I went the direct route, and stated my own case. I made no headway. Then (and this was highly misguided, but I was 19 at the time and extremely naive!) I tried to prove that she was a liar. I got stone-cold proof, in her own writing of a colossal and important whopper she'd told, which was the whole basis of an appeal to my parents for support. Despite the evidence, I was told (not without justice) that I was interfering and upsetting her.
The pain and the depression from feeling this rejection and not being able to do a think about it were awful and had a hugely negative impact on my life. And then in my mid 20s I went to counselling, and I got the courage to distance myself and pursue my own life, which of course went down very negatively with my parents.
I still make an effort. I ring them every week, even though they make it clear that they are bored of my company and are merely tolerating my calls. Every time we speak or write, I make it clear that they have an open invitation to my house. They have come four times in the last 18 years.
Now here's my point: if you asked my parents why they did this, they would say that my sister has support needs that far outstrip mine. In actual fact, I have suffered serious illness, homelessness (literally living in a tent), abuse and divorce by myself, while she's always been comfortable and has grown rich on the back of their labour. So I am acutely, painfully aware of the fact that evaluations of 'need' are incredibly subjective.
I think the best thing to do is for families to discuss these issues openly. But when you are dealing with someone who is not behaving according to the 'normal' rules, and who has colossal power over the interpretation of all events, that's not always possible. Which is why I would argue throwing all of your resources behind one family to the neglect of another is not necessarily a good plan - because even in close-knit and functional families, you can't simply assume that the needs of an adopted child are greater than the needs of a non-adopted one, or that the person who shouts loudest really needs more support.