Told my folks today... it went really well! Certainly better than I expected and they are both understanding and supportive (we'll see if they still feel that way when the shock wears off).
I explained to them about my melt down in February last year and that after discussing it with medical professionals it became clear I still had "gender dysphoria" (don't like the phrase myself, but it helped to frame it for them). I stressed that I loved them, that I didn't arrive at the decision lightly but I couldn't continue to live my life as a man that I needed to live as a woman. My mum held my hand the whole way through (especially once my waterworks started flowing, and they were really flowing at this point!).
But they listened as I explained what I was doing, that I would be starting work next week as Grace, that work and friends were very supportive, that I passed well (& showed them pictures to prove it!) and they never once got upset or angry or ridiculed anything I said. My mother had to acknowledge that as a child I had never liked playing rough games with other boys, and that she had kind of wondered if I had ever really "given up" on my first transition. My dad was pretty stoic but said he was completely supportive of whatever I decided to do. He even let me hug him!
My mother said "I guess I have two daughters now" and I said "you always have" and she smiled and nodded. So happy!
So I will be going to my niece's birthday gathering after all (it's actually a joint thing for her and a belated one for me). I will be going in guy mode though as my sister and kids don't know yet, will talk to her at the end and then decide on the best way to tell the kids if she is open to it. Knowing my folks are supportive might help her feel OK about it.
Phew! What a massive, massive day!