How do you handle siblings that won't help, but tell you what you are doing wrong in caring for parents with Alzheimer's?

Clearly they are too stupid to give a REAL response to, so you can just shine-them-on.
You say:
"It is SO kind of you to always think of us, and to always give us such stellar advice. You are wonderful, always thinking of others, always with their best interests at heart. I am going to seriously consider all that you say just as soon as I find a little space to come up with for air. I am always ASTOUNDED at your ability to give the BEST advice. The best help any caregiver can have is the advice of friends and family. I mean SOME people would rather have an elder-sitter for a few hours so they could get a haircut or someone to go to the grocery store for them. Some would rather have a casserole of something. ME? I love advice. I mean, basically, I just live for advice. I hope that if you think of anything else I can so you will call me day or night. Now, if I don't answer it may mean I am busy, but I will be certain to call back just as soon as I can come up for air."

You can have fun with it. If they get offended just look innocent and say "I am perfectly serious!"

Of course, stupid as these folks are it's real likely that they will believe every word you say, and call you at midnight with further advice.

When they do this say " If you think you can do a better job, then you can do the caring".

When they suggest something different than what you are doing …… you ask your sibling …..
” When will YOU be doing that for Mom and Dad ?”

My first reaction is to say Back Seat Drivers either shut up in the back or get out of the car..

But a more thoughtful response may be to ASK why they are suggesting what they are?
What do they SEE?
What CONCERNS do they have?

There can often be a *knowledge gap* on the health issue or the situation.

Eg D is only eating protein shakes - he needs a good steak.
M looks all crumped in stale clothes - always was very well groomed before.

Out of town or background sibs may knoe KNOW that D refuses all other food or that M refuses to wash & change clothes.

Tell us more if you think it can help.

You ignore them. The ones doing the work get to decide how it’s done.