I find it very difficult, too. I think you have to develop a thick skin, and learn to keep your own irritation under the surface. Just answer the questions they ask, in simple terms, and try to keep a sympathetic attitude. I work in ICU so if things get out of control, I can have family removed. I've never had to do that, but in some situations, the family nagging can remove my focus from the patient.
Sometimes it all comes down to trust. The family is stressed, and they kind of feel you out to see if they can trust you. If you can remain calm, they may be more at ease. I'm pretty willing to let a lot of things go when it comes to stressed families, but I would never put up with outright rudeness.
I try my best to make the family feel better, but sometimes everything you do will be wrong. If you have a good supervisor, it may be worth it to involve them in the situation. Sometimes having someone in "authority" backing you up will take care of the situation once and for all.
Then, on your way home in your car, you respond to their rudeness the way you REALLY wanted to at the time!!! I do it all the time!