I agree with job here. Why are you not going to go ... allowing your W to make these events uncomfortable for you, I know it stinks and the anger is there .. all that I get it ... but honestly wouldnt it be better of you to go inspite of her, go for your S and make your W and OM uncomfortable ... this is YOUR family not his. Let them feel awkward ... this is YOUR turf and your son ya know?
Do it with grace and confidence ... you deserve better than a TM 'heads up' .... that just shows her character right now ... she is full on in Lala land