I have a feeling I will be told IABU but here goes as I need to get some perspective.
To cut a long story short, I absolutely detest my SIL. I never really got on with her much but tolerated her for 'family'. I have come to realise that she is toxic and I suspect narcissistic. Following a 'subtle' hate campaign against me on FB and some other pretty bad stuff, I have blocked her and now refuse to have anything more to do with her.
She is very self centred and has on occassion gone out of her way to spoil events for us or our DC. I have posted before about these and been told IANBU to suspect she does this purposefully.
I'm not sure why, but she insists on turning up to our DC's birthday parties. I never go to hers, I just let STBXH take the DC because we dislike each other and there is no real reason I should be there. Whenever she is around me, I feel all tense and nervous and scrutinised by her. I can't relax and it puts me on edge and therefore I cannot enjoy myself. The party is at a playgym so hardly 'fun' for an adult.
It is DC's birthday party coming up and I have asked STBXH to tell her not to come. He is refusing despite the fact that he dislikes her also. He is telling me that I should tell her, so I'm considering messaging her saying something like, 'hi x. Given the situation between us both I think it would be best if you let BIL bring DCs to the party'. SIL is not quite yet married to STBXH's DB if that makes a difference.
If IANBU, is that message ok or what can I say that is polite but gets the message across that I want her nowhere near me or my DC?