Personally, I think parsing the exact wording of text messages, to discern whether they are one word away from being PA or not, is looking for trouble, and if you look for offense hard enough, you are sure to find it.
I vote with those who find SIL's disappointment honest and blunt, but not necessarily PA. I also don't think mentioning the Nephew's feelings is necessarily manipulative. A 5 year old who loves Auntie Green Bean and has been looking forward for weeks to have his family hear him sing, is no light matter. He is 5, it's not like he can speak for himself in this conversation.
I would also like to point out, that since you seem to be intentionally calculating a way to NOT see them on Christmas, by offering a non-alternative (maybe I could come to your house but secretly I'm hoping that will be too late), your SIL is actually spot-on. You don't want to see Nephew sing, you don't actually WANT to see them at all.
You are allowed to not want to go, that is perfectly OK and your choice. She is allowed to be disappointed about that, and to tell you so.