Half-sister traced my dad. Mum and Dad met up with her, everything was OK at first. Mum very supportive, as Dad had told her about existence of child when they first met, so it wasn't a surprise. In fact Mum very nice indeed about it, very sympathetic.
Siblings hadn't known about her existence, but met half-sister for lunch, and she seemed more or less OK, everything quite civilised to start with. However brother couldn't stand her and wondered if she was after money from my parents, as she was a bit money-orientated generally and more so than us. I was more neutral about it.
Dad then became a bit obsessed with catching up with the past and started ignoring the rest of us.
Mum then found out dad had been trying to trace half-sister for some years without discussing it with her. Big row. Mum left dad briefly. Muggins caught in the middle trying to do marriage guidance - not at all funny.
Mum returned to Dad and things settled down. Parents saw half-sister a few times, but eventually she had a major psychological wobbly and blamed my dad for her entire childhood. As he was never consulted about the adoption or told her mother had been pregnant, this was very unfair.
Mum and Dad decided it was too heavy seeing her, so told her they wanted to stop.
Half-sister was OK with this but effect it had on Mum and Dad was to make them very insular and 'coupley', so they don't see the rest of us much either. Mum very moody and allowed to get away with all sorts of manipulative nonsense, as it was Dad who used to bring her in line, but he daren't now.
All in all, everyone miserable. No counselling for us poor siblings, only half-sister. Nobody ever gives a thought to half-siblings. She always knew she was adopted, we didn't have a clue she existed, so it's been a lot harder for us.
Have told half-sister a lot of this as nicely as I possibly can, because I don't have any particular beef with her, but she still feels guilty for the damage she has caused. Although I think she's nice, I don't talk to her much because I don't want to get reeled any further into what amounts to something like a TV soap opera with all the drama and emotion involved.
As I say, it's a can of worms ...