This came up last year, and as time as has passed, I only feel more strongly about it. I think you need to tell people what you are inviting them to, whether or not it's traditional! Because even if it is traditional, your tradition might not be exactly the same as someone else's tradition, and that's not always likely to occur to either the hosts or the guests. Whether it's football versus the Macy's parade versus another activity, or canned cranberry sauce versus fresh or turkey versus the family tradition of serving lasagna for Thanksgiving. You should give guests a brief rundown of what you're inviting them to, before you ask them for a response.
So I think DH's cousin should have said, "How about if I cook Thanksgiving dinner this year? And I can do a whole Asian theme, with wasabi mashed potatoes and a hoisin glazed turkey and a spicy ginger/apple pie and then we'll all watch football! It'll be awesome! Would you like to come?" And then the potential guests could say, "That sounds very creative, dear. But I'm afraid Grandpa and I wouldn't be able to handle the spiciness, so we'll have to decline. But we hope you have a great Thanksgiving and we'll see you at Christmas!" And if DH's cousin is more invested in the Asian theme, she can have it without some of the family members. And if she's more invested in having the whole family together, she can change her menu. It's still up to her, but what she chooses could have consequences.
Since she didn't let people know in advance what kind of event she was inviting them too, I don't think it would hurt to casually spread the word to a couple relatives in a conversational way. "Oh, I'm so sorry DH and I aren't going to be able to see you guys for Thanksgiving, it's one of my favorite holidays! And Cousin's idea for having an Asian theme for Thanksgiving sound really interesting, but I don't know if the wasabi mashed potatoes will be everyone's cup of tea? Anyway, DH and I are going to my relatives this year and I always like to make the pumpkin pie...."
I still like traditional Thanksgiving foods, but now I think we'll be having Asian food for dinner tonight!