I am tempted to say ignore her, for your own sanity ... but I also believe that ignoring racism permits it. Your response doesn't have to be long, and you don't have to include all the reasons her email is well, the dumbest thing you've ever heard but I don't think you should just let it slip by.
My twin sisters in-laws are notorious for these kinds of comments. They're not even MY in-laws and they drive me freakin nuts. Her step MIL informed my 5yo nephew that Turks had bad hygiene and scratched their behinds all day long (my DH is Turkish). At my niece's first birthday party, I had made my DS a special outfit for the occasion ( see it
here ). My Mom commented that it reminded her of traditional Middle-Eastern clothing, like a tunic. My sister's Uncle-In-Law remarked that "yeah they wear pajamas all the time" with a very negative tone.
Anyway, comments like that are ridiculous, uncalled-for, and probably not what you want your DC exposed to in the future. Best to try and correct them now, or at least make it clear those types of stereotypes are not acceptable and hopefully they'll keep it to themselves from then on.

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