Sibling tells dementia parent terrible lies. Any advice?

The sibling says these things to drive a wedge between you and your father. Then, her and her son will go into your parents house (with your fathers permission) and haul off 3 trailer loads of your parents belongings. That is her motivation. Dad wont remember any of it.
It will destroy the loving relationship that you have always had with your parents because now dad asks about your sibling who has been a lifelong heroin addict and for the first time in your life, you really miss your dad.
Your life will be destroyed when you are sued for financial elder abuse which will have a devastating affect on your emotional well-being, so much so that you spiraled into a deep depression. As a result your business fails because you are unable to work & you run through your SEPIRA to pay your attorney and to live on.
You have lost everything & dads wallet is $50k lighter.
It was the free senior centers lawyer who started it and without questioning me at all he wove this horrible tail of deceit the likes of which would never have even entered your mind. Then he passed my poor dad over to his son, an attorney for hire to sue me and take dad for $50k. Dad doesn’t remember that either:(

What does your niece have to say about this? She is your father's primary caregiver. Evidently she is fine with your visiting your father, no problem there, so we can assume that she at least is not determined to damage your relationship with him. She is also the person who is primarily responsible for his welfare, and if anything distresses him she could try to put a stop to it. How much influence does she have on her mother? And how does she feel about your visits? And how do you get on with her?

So. Your father lives with his granddaughter. Her mother lives nearby? - and visits him frequently? You live out of town and visit less frequently; and when you do visit you hear from him that upsetting and nasty remarks have been made about you, presumably as part of general comments about your impending visit.

Your father is sad about these remarks. There could be all sorts of reasons for that. He might, for example, be sad that his daughters don't get on and that one says spiteful things about the other. Do you think he even half believes what she says? If he really did believe that you were only there to lock him in a nursing home and steal his money, would he say so to you?

I don't think you'll get very far with trying to make your sister be less of a cow. Isn't it likely that she will respond with "I can say what I like to my own father" combined with "truth hurts, don't it!" and a range of other self-justifications which will only make things worse between you?

You can work on how you go about reassuring your father. It's the way in which you "discredit the lies." Look on it more as ignoring the lies and presenting him with the self-evident truth, which is that here you are, not taking him to the Nursing Home today anyway! And he's pleased to see you, isn't he? - so that's one person who doesn't hate you. And his money - you'll be happy to take it off his hands if he insists :) - but perhaps he'd better hang on to it for now.

And as soon as ever you can, steer the subject of conversation into pleasanter waters such as that you've come xxxx miles to see him and how much you've been looking forward to it and how happy you are to be with him.

By the way. Setting aside any conversations you're having with your father himself - do you, in fact, feel he would be better cared for in a memory care unit? Have you ever said so? Is that particular family discussion ongoing?

If the parent has dementia, their statements may be based on fantasy. My mom can't remember anything that is said to her, not even 5 seconds after it is said to her. She says she sees "dad" outside her window (he died last year.) She concocts other fantastical statements. It's because her brain is damaged. She doesn't know any better. It will not change and we will not be able to "correct" her thinking, such as it is.

Sibling A doesn't like Sibling B. Sibling A's adult daughter is the primary care provider for the parent (parent living in her home). When Sibling B comes to town to visit parent the parent will say "everyone hates you", "Sibling A said you're here to try to take me away", "Sibling A says you're here for money". Ridiculous stuff like that. Sibling B is more concerned about the effects these lies have on the parent.
And unfortunately when Sibling A is told that though these lies may seem funny/wise/amusing to themselves in reality they truly only hurting the parent.

Reply is "deal with it". Just wondering if there's any constructive way to deal with this behavior other than ignoring it.

The sibling is very forthcoming in acknowledging this behavior. Then the parent repeats the information. Its very sad.

I’m wondering how you know this. Do you personally witness this? Or, does your father tell you? If your father is telling you, remember he had dementia and he might not be too trustworthy to repeat what he was told.

Is sibling primary caregiver?  Need more information

Like what?

What sort of lies?
Why would the sibling think it wise/amusing/a good idea to tell the parent lies - what is the sibling trying to achieve?
What effect are these lies having on your father?