Why can't you clear up the misconception IF ASKED and still keep your mouth shut about disliking your in-laws? There's is no good that can come from complaining about your in-laws but plenty of misguided anger that can come when people think you're prejudiced about something.
When your sister-in-law says, "she gets it" or someone asks you why you don't like the elderly, you can defend yourself and simply say you're not sure where that assumption came from and that you don't dislike the elderly.
If they take the questions further to say, "well then, why don't you like Mom and Dad?" again, you can say you've never said that you didn't like them. Admit that perhaps there are some actions you don't agree with (their avoidance of anything new) but you understand that they're just different from you and that's all it is.
Oh, and I forgot to include... if your husband can't handle criticism of his parents and keep it private, you shouldn't talk to him about it. Keep your criticism to action items like, "I wish they would be willing to try new things or I wish they would respect the rules in our house" but avoid personal attacks. How would you handle someone attacking your family?
And try to have some empathy too. Are they refusing to remove their shoes without explanation, after you've asked them specifically to do so? Or are they simply not aware that this is an issue for you? Perhaps they have a hard time putting on their shoes given their advanced age and they don't want the embarrassment of taking them on and off in front of other people? My own mother is not this old but has bunions on her feet and is mortified of people seeing her feet. She of course would never admit this when confronted with the request to take off her shoes... And I personally have plantar fasciatas (sp?) and it pains me terribly to walk without my shoes, but I have no problem explaining this and asking for "permission" to stay in my shoes when in someones home. Just a couple of alternate positions for you to consider.