For future reference, it's best to let someone know that you want them to bring something when you issue the invitation.
That said, because you know that she enjoys making salads and you are family, I don't think you'll be sent to eHell for making the request.
I'd ask your hubby to ask his brother or SIL, and be sure to mention how much you love the salad. "Joraemi has been waxing poetic about your scrumptilicious salad. Do you think (SIL) would be willing to bring one for Thanksgiving?