Defamed online - Do Cease and desist letters do anything?

Location: Michigan

I am being defamed online (Facebook, somewhat frequently) by an estranged and mentally ill sibling. They are extremely deranged, use my full name, and are accusing me of serious accusations that have legitimately never occured during our childhood more or less (witholding specifics to keep a degree of anonymity here). The sole purpose of the posts are to hurt my reputation and inflict emotional pain. Essentially, its a lifetime of one sided sibling jealousy being spouted off to roughly 1000+ followers in a disgustingly, horrendous way.

The curve ball here, is that due to the mental state of my sibling, these accusations are believed to be true (by them) and continue to grow in severity (think of fish story, but there wasn't even a tiny fish caught in the beginning). I should add that mental illness has NOT been officially diagnosed due to the refusal to never seek psychiatric care on their part. But if anyone were to know the intracies of this person, narcisstic and psychotic parallels have been glaring for the past decade.

I've spoken with 3 separate lawyers (2 of which are in my family) and the consensus has been consistent throughout: you are dealing with someone who is mentally unstable, attention seeking, and angry, and it's best to let it be.

I am aware this process begins with a cease and desist letter (obviously, requesting all posts be taken down and no further posts made). What I would expect with near certainty, is the posting online of this letter by them, and the continuation of defammatory posts.

I really don't have the time, desire, or energy to take my sibling to court over this and also fully expect a counter sue of which I am now going to court as the defendant. Despite having done absolutely nothing, the idea that I have to take the time to defend myself over something completely made up (there are no grounds for anything against me) over god knows what, is less than appealing. We are in different states as well and I understand traveling to eachother's state is required too. I have a good job, a family, clean record, etc...messing up any faction of this is my siblings main goal, sadly.

Am I thinking correctly that I just have to take it on the chin? I've been doing this as a deterrent to kicking off a gross, legal cycle. I don't want money (my sibling has none), but I'd be lying if I said this isn't affecting me emotionally/mentally, and also fear my company can get wind of the posts. While 10000% untrue, it's a he said she said thing, right? As far as I know, I have no leverage with Facebook as far as requesting the posts be removed.

Any help/advice/experience is appreciated. I'm in a really unfair (and crazy awkward) Position. Thank you to anyone who took the time to read this.