As the old saying goes, you can pick your friends but you can't pick your family. There must be people like that in every single family!
If the person standing up for you is doing it out of choice, you might want to speak to that person (privately!) just to thank him or her for taking your part. It can be so hard to be on your own, without anybody on your side. Be careful what you say - don't let it turn into a backbiting session.
When I have been in that sort of situation and can't "humor" my way out of it, I tend to say nothing, too - if I'm the one who is being attacked. When somebody else is attacked unjustly, I will stand up for that person.
But it's no fun, any way you try it. Sometimes confrontation can be effective. It depends on your nature and the nature of the person you're confronting. If that person shifts ground or is just too nasty, I can understand your trying to take the high road by choosing not to say anything and letting the ugly words be the problem of the person who is saying them.
If you decide to keep doing it this way, you must be *sure* you don't stew about it, and don't let any bitterness develop inside you. Bitterness (unforgiving anger) will affect your life, your husband's, your children's, everybody's except for the person who is causing the problem.