I know this is passive aggressive but I got some enjoyment thinking about it.
perhaps you should make your response look like an "auto generated reply"..
"Your original message to "insert your email address here" has been deemed undeliverable for the following reason:
it's absolute crap"
tee hee...sorry. But come on. How condescending and manipulative can she be? I agree with the other posters....tell her you've already made plans and possibly add if your siblings would like to spend time with your children they are well aware of your phone number and can reach out themselves. Wish them a happy holiday and then set about enjoying yours.... guilt free....She is obviously trying to get a rise out of you...the whole email stinks. But nothing changes if nothing changes. Be the change.