Hi Fraidy,
Yes, the whole family dynamic is really weird, isn't it. My younger sister is now in the position I used to hold. On the surface it seems that she gets all the help, support and praise - that's how it looked on the outside when I was younger and I was the one that seemed to get all the good stuff. But it is conditional on complete obedience, a complete absence of your own personality and the need to construct your whole life around her and the fantasy that she lives in.
I didn't understand any of this stuff when I first stood up to her. All I knew was that I voiced my opinion a couple of times and told her I wasn't willing to put up with a number of things anymore and I unleashed a decade of hellfire and brimstone from her. I think maybe it's mostly because reality rocks the fantasy she lives in and she can't cope with that - I think the reality of her life is too scary for her to contemplate. So in the same way that I would fight to the death to defend my son, I think she's willing to fight that fiercely to cling on to her version of what is true. She's always believed that my step-dad and she are in this amazingly powerful, loving relationship, that he chose her over all others, that their love was so strong they couldn't fight it and various other nonsenses. In truth, he never stopped sleeping around, she just chose not to see it, just as she chose not to see him abuse her kids.
Sometimes I feel sorry for my younger sister, sometimes I despise her. Her house is identical to my mums, right down to the paint on the walls and the colour of the sofa. She couldn't afford the same car as my mum but bought the one nearest to it that she could. As you say about your NSis, she was never very well liked at work and was known as a grass - other staff would cover for someone if they'd nipped out for a ciggie but she'd go and find a manager to tell them. She seems to delight in getting one over on someone. My mum was always very like that as well.
I love the idea of having a wedding in an art gallery! Will there be anyone else there who you know will be alright to chat to? I always had an uncle who would happily chat to a plant pot if necessary and kept out of the entangling that goes on. But being able to wander around looking at pictures will be a good distraction either way.