Perhaps I'm naive, but I honestly don't think it's anything sexual/romantic. She's 15 years older than him and seems very happily married.
She has hated our relationship from the start - we were friends beforehand and us getting together changed the dymnamic of the group, which has more or less fizzled out now. We did consciously try not to behave differently with the group, but obviously we started doing things as a couple rather than always as a group. I expect the change to the dynamic was inevitable?
She has told me she doesn't think he's good enough for me - she thought I'd be with someone professional.
She often does things to take his attention away from me. She's very loud and much more outgoing than I am, loves to be the centre of attention, and he gives that to her.
I've realised looking back on my own friendship with her, that you can only be her friend if you do what she wants. E.g. she took great offence because I declined an activity she wanted to do, but she didn't come to DP's birthday because the thing he had arranged (that he enjoys) was "boring".
She is constantly driving a wedge, persuading him that the thing she wants is more fun than then thing I want....but he lets her.
We are great together when it's just us but awful whenever she's around.
Is it me and some unrational jealousy, or him and a weird fixation?