My SIL is amazingly hard work.
She was invited for our first Boxing Day at our first house together with her dh (DH's brother) and then infant son. She accepted the invitation but then went on and on and on for months that they may not be able to come because of his routine, they wouldn't stay long if they did, etc.
On the day, she didn't turn up. BIL came and brought the baby. They stayed all day.
Months later he let slip that the reason she couldn't come was because we lived in a house (bought it that November) and they lived in a small flat.
A couple of years after that I turned 30 and invited everyone to a James Bond themed party. Not everyone cup of tea but I wanted cocktails and glamour. Everyone wore black tie. Except SIL who came in a track suit with unwashed hair and no makeup.
The bizarrest part of that is that she is very very polished the rest of the time, never without makeup and accessories. I never found out why she did that.
She declined the spa part of my hen do saying that the £60 was too expensive. Which I was totally fine with until she met us in the evening for the pub part and went on and on and on at great length about the new top she was wearing which had cost her £60.
And then she turned her back in the group photo.
There's loads more.
I have distanced myself now.