Golly what an awful text.
It's fairly obvious that this isn't something you'd threaten against someone you supposedly love.
So why do you think she sent it? Especially sent it to you?
From what you've already posted she's used this tactic before "do as I say or we will split up" - but previously she's done it only to your son and it worked.
Presumably your DS has told her about your NY plans.
What we don't know is what he's said to her or what he really feels about it. He could be complaining to her that you are "making him" go away and he really wants to be with her.
However, there's also the possibility that he's "glad" to blame you and it's a good excuse not to be with her.
Either way, she's decided to intervene using the tactic that's worked before - only on you this time, so if they spilt up it's your fault.
So your next steps....
It's very tempting to respond, but I personally wouldn't. It's such a provocative text there's no honest response you can make thats not going to add fuel to her fire and play along with a narrative that you are trying to split them up (we've had plans for months and you're ruining them so if I go off with someone else you're to blame).
I'd also bear in mind that your initial text was very reasonable and if things do escalate and you have to involve 3rd parties you want to ensure all the "evidence" you can present in is your favour - which right now with the texts it is.
So the question is whether to show this to your son or not.
When he gets home I'd ask him "lightly" if he's had any further thoughts about NY and see what he says.
What you need to try a gauge is what threats she's made to him and what he feels about it.
Overall I'd be tempted to say that you'd love to spend the holidays as a family but you understand that he is a young adult now and might want to choose to spend it with his GF....and that's fine as long as it's his choice.
Then show him the text and ask him what he thinks about it. Don't comment on it first yourself.
What you need to try and do is prompt him to reaching his own conclusion that some who claims to love you wouldn't behave this way and "swap you" with other boys simply because you wanted to spend time with your family.
So ask him leading questions. Why do you think she sent this to me? Has she said something similar to you? What do you think about what she's said?
Try and keep your questions as neutral as possible. Don't for example, say something like "do you think she's being manipulative/threatening?".
Ultimately you want him to reach his own conclusions.
Good luck 
@CoraPirbright thank you 