Difficult stepchildren, any advice?

I so agree with others. You already know you give good and loving care. YOU know why you do things as you do them (and to be frank, what the problem is with one beer I cannot imagine).
I would ask one question. Were the step kids ever any different? Because I would be surprised if they were.
I would remove myself entirely during your visits so that you are not a party to "questioning looks". I would usher them in, welcome them warmly, ask if you can bring them a snack while they visit, and then take myself upstairs to watch Better Call Saul, Breaking Bad, or something else with a lot of murder and mayhem. Tell them to please knock on your bedroom door or text your cellphone when they are leaving. Wish them a wonderful good night and tell them you look forward to their next visit, and you know their Dad does as well.
Big SMILE as they exit the door. I know my lifelong experience has taught me one thing that is CERTAIN and set in STONE. Nothing bothers people of this ilk so much as returning kindness and bright smiles. They loathe it.
Have fun with it. In fact, have a ball with it, and when they leave you will have the next week to enjoy the sour looks on their faces as all their nastiness gets them nowhere.
Best to you.
(You could even say "You know, if you ever have ANY QUESTIONS, I will be THRILLED to answer them for you". It's called "passive-aggressive" but you leave the aggressive out. It will drive them nuts. It will be THEMSELVES after the beer.)

Here's my advice: when they come to visit their dad, leave them alone with their dad. Do something other than sitting there waiting for their sideways glances and accusatory questions. Be unavailable. Go for a walk. Take a hot bath. Take a nap. Call an old friend. Knit. Read. Watch a movie. Cook. Bake. Do something that makes YOU happy. And start doing it as soon as they get there. Be cheery about it too a la "So glad you're here. He's waiting for you. Help yourself to a beverage from the fridge. I'll be back in about an hour" then make yourself scarce.

Since they don't care about you, do not give them an opening for anything. You *tell* them what's what because it's *your* house.

Next time they come over is when you conveniently have a medical appointment or something they cannot challenge for a couple or four hours. They pick on you so much, let THEM sit there and do MC care for a couple hours.

Tell the step children that if they are unhappy with the care you're providing their father with, then THEY can come care for him instead! Or, if that doesn't work, they can arrange to have him placed in a Memory Care Assisted Living Facility where they can go visit him for an hour 2-3 times a year.

In the meanwhile, you should suggest they read ALL they can get their hands on about dementia, what it looks like, feels like and sounds like, so they can pull their heads out of their buttocks and THANK you for the AMAZINGLY EXCELLENT job you're doing caring for their father while they're sitting around doing NOTHING but pointing fingers!

Armchair critics are The Worst Of All, aren't they? They need to walk a mile in YOUR shoes FIRST and THEN call to complain. They'll be kissing your feet and sending you 5 dozen roses as a small token of thanks for all you DO.

Sounds to me like you're inferring what their questions mean rather than knowing it. "Sideways dirty looks?" Trying to see if you're not in quarantine? Come on. Maybe they're just trying to make conversation with someone who doesn't make a lot of sense.

How about having an actual conversation with your step-children instead of assuming what they're thinking? The step-relationship is complicated even in the best of situations, but if you could manage a non-hostile conversation with your husband's children, you might gain some allies.

I know my mother had a tough time with knowing how to be helpful to her stepmother as my grandfather was dying, and as a result, she didn't get to spend much time with him in his last months. My step-grandmother was only five years older than my mom and had come into their lives just a decade before, so everyone was an adult when this marriage occurred. She was a nice lady, although she didn't fit in with the family terribly well, but everyone got along. Still, it was tough to know as an adult child where my mother stood. She loved her father, was still devastated by the death of her mother, and she felt she wasn't able to have a close relationship with her dad because there was a new wife in the picture.

Just talk to each other and try to stop imagining motives that likely aren't there. Assuming the worst of people is rarely useful, and remember, he is their father. Try to have some respect for that relationship.

Sorry, but I am big on gut instinct. And I don't trust people that show up once a year or so and think they know better.

As said, walk out of the room. Take what they say with a grain of salt. The worst they can do is call APS. APS will find the accusations false. You have Drs to back you up. You have his POA. The ball is in your court. If the question is asked "why did you tear the page out of his book". Just tell them that didn't happen. This is how Dementia works. Their reality is not our reality.

If you have never had a relationship with the steps, you probably won't now. You are the 2nd wife and may be seen as a threat to their inheritance and as POA they have no control. As said, you know you do a good job. Thats all that matters.

I am only assuming, but sounds like you may have never had a good relationship with the children. It's not going to change.

The next time one of them questions something like beer, just say - you do know he has dementia, right? We don't keep beer in the house. The hard thing about dementia is that the patient can sound very convincing at times because what they are saying is very real to them.

My boyfriend's mother accused her husband of hitting her. He would just sit there and not say a word to us to deny it. We actually thought it might be true because she was in the beginning stages and we had no idea her mind was going. Later on, we found out she was the aggressor and he was reluctant to tell us because he promised to never put her in a home - he thought we would do so. We had to make sure the police were aware she was the one doing the hitting and scratching in case she called them to the house. Being it was in a small town, the police knew the family. We eventually had to move her after convincing him he had done all he could for as long as he could.

Did you ever have a close relationship with your stepchildren?

If his children weren’t close with you before your husband’s decline then they can’t be expected to have a warm and fuzzy relationship with you now.

I am sure that they aren’t pleasant to be around.

Have you tried speaking to them about their dad’s situation?

As someone with a stepmom, I too was concerned through my 20’s-50’s about my rightful inheritance. I simply do not want my Dad’s hard earned monies to end up w/ the adult step children that he never raised. They’re nice people, I have nothing against them. We were all adult children when the 2nd marriage occurred and we live all over the USA, so no close relationships.
That being said, it took me 20 yrs. to realize that my Dad’s 2nd wife took loving care for him and would be the one to see him through the end of his days. As it turns out, she came down with Alzheimer’s at age 79. My 87 yr, old father is now her caregiver. Nothing has turned out the way I thought it would. This is life. Now that I’m 60...I’m grateful that both sets of parents as well as MIL (all 3 moms have dementia) have the funds to provide for caregivers, AL facilities, etc. I would make a terrible care giver. I live far away from both my mom and step mom. I SO appreciate my sister who cares for my mom. I support her in whatever way I can. In regard to my MIL w/ dementia......she was abusive to me for decades. I feel no responsibility to help additional abusive family members out with her. When I do, it’s out of the goodness of my own heart. I have no interest in her medical r3cords, meds, etc.
I don’t know what your situation w/ the 2nd marriage was, but kids are wrecked by divorce...even when they’re adults. We have latent anger issues, abandonment issues that we need to work on, especially if wife #2 was cheating on our mother with our father. All we have left is a possible inheritance....a small proof that our Dad loved us.
I have no answers, but offer a perspective.

Such excellent ideas and suggestions here ~ particularly like the ones about not letting step kids comments annoy you (YOU know what you are doing and so do H's doctors) and not giving step kids an audience on the few occasions they decide to drop in.
As an aside, I will state again that I am constantly surprised by the number of people (mainly children) who automatically assume they will inherit something from their parents particularly in cases of second marriages or blended families. And frequently this assumption is done in families that are not even in the 1% of wealth!

Already have informed my son that I'm leaving what little I have to my favorite charity to care for the elephants. His response: "Well, that's a good idea Mom. I think I'll look into that also. You've given me everything I need to survive and thrive as an adult. I don't need anything else but your enduring love."

I gotta love that kid!