Tbh it just sounds like you are itching for any old excuse to cut contact.
The party not dramatic enough for you?
Presumably you aren't really over whatever the heinous crimes were that the committed earlier in the family saga, and so you aren't really capable of the level of detachment that you have insisted upon. That is YOU changing the mutually agreed rules of engagement, not her. She didn't boot the child, or spit on his head, or tut and sigh and move away - she just didn't coo or interact. Make your mind up. It sounds as though you aren't up for the maturity involved in this 'tolerating each other at family events' stuff.
Do you need to have a really good outpouring of hatred and a formal announcement to the whole family that you are cutting contact because of the terrible terrible things she did? Do you feel like she's got away with the terrible terrible things too lightly, and so you need to remind your family about the terrible terrible things she did?
It all sounds like a fuss about nothing, tbh. She just didn't play with your kid, that's all. Probably terrified you were watching her like a hawk in case she touched him and infected him with the terrible terrible things she did...
She might post later and say 'my sister hates me with a passion but we tolerate each other in family company so as not to cause a row. Her Ds was rolling around under my feet at this family do today, and she was staring at me like she thought I was going to stamp on him. I was terrified she was going to cause a scene so I just sat there until he rolled away, but I think I've upset her even more now... No idea why, I didn't touch him!'
By the way, the glee and obvious validation with which you reacted to vampy's post was extremely unedifying, and said more about what you were really seeking from this party than your op. You couldn't have cared less what she did with your son, you were just looking for an excuse to lose the tolerating each other thing and find an excuse to let her have it. At least be honest and say 'I haven't really got over what she did, and I don't think I can be arsed to stay in contact for the sake of our family'.