What am I to do about Mom's loss of memory?

Those with a more normal mind should prevail.

Hopefully you read the booklet I linked you to not long ago, Understanding the Dementia Experience?

Once you learn more about the condition, then you'll realize paranoia and all that your mother is doing goes with the territory, unfortunately. She will likely continue to accuse you of 'wrongdoing' which doesn't mean you are doing something wrong; just that her mind is broken and she cannot remember what's happening from one hour to the next, which makes her BELIEVE she's being tricked. The best thing you can do is not tell her what's going on with ANYTHING until the moment before it happens; then tell her what's happening and get her going to whatever place she needs to go to. Otherwise, she'll dwell on it or question it and drive YOU to the brink of insanity with questions, accusations and generalized anxiety about it! She doesn't realize her doctor diagnosed her as 'incompetent', nor should she be TOLD that, either. In her mind, all is well and she's in charge of her own life. Who wants to know such a thing, or believe they're 'incompetent' to make their own decisions? I think it's human nature to reject such a notion and for the mind to make up another story to compensate for it, don't you? I think I'd do that, were I to be told I was incompetent, or realize my children were trying to run MY life FOR me! LOL

I know how awful all of this is for YOU to deal with; my own mother had advanced dementia for a couple of years and was tormented with various notions that her siblings were still alive & trying to get to her but were being prevented from doing so. It was a horrible thing for ME to try to deal with and to try to help HER deal with, more so. I had a hard time coming up with stories about where the siblings were and why they 'weren't able' to come by today. Or where her mama was and why she couldn't reach her on the phone. Just a wretched situation for all involved with the care & management of a loved one with dementia.

Wishing you the best of luck coming to terms with the fact that you aren't The Bad Guy here, but trying to do what's best for mom, whether SHE sees that or not.

Paranoia is common. It will come and go. She will distrust you this week and next week it will be your sister,
I would suggest though that rather than Assisted living she be in Memory Care. Most Assisted Living a resident can come and go as they wish, just signing out before they leave. (If they are seen leaving) A "walk outside" can lead to getting lost and wandering away.
And even if she got to the lawyers office a lawyer would not find her competent to change any papers that she has previously written,.

Nothing. You can expect this. Paranoia sets in. Just continue to do your work. For my brother I sent a monthly accounting that I had to keep anyway, of every penny in and every penny out. I also made him his own checking account; we went together to his bank and set it up and he sat and said to the manager "I know it seems silly I want this" and they were all reassuring to him. He only grew that account before his death. So I kept him informed as though I was his acocuntant and he never descended into paranoia to worry, though he did have issues thinking people who had access to his room would "steal" from him, were watching when he signed out for his walk and so on. Just explain to sis that this isn't unusual, and she should reassure Mom. I am hoping your Sister is not a whacko, but if she is and is considering this, then get the guardianship in place. She won't get that set aside; in all truth she won't get this set aside either. And if she is a whacky one do let her know it would be illegal for her to take an incompetent Mom to an attorney and mess with this set up.
There's enough on your plate.
Much of what you must do about this depends on how sane or insane your SISTER is. Sorry, but that's about it.
And by the way, messing with a TRUST? Difficult to do to say the least and no lawyer is going to want to risk his license on THIS one. When my bro and I went to make me Trustee of his Trust the attorney cautioned him up one side and down the other and said to him "You are sure you trust this Sister with your entire life, because she can sell the gold out of your teeth if you do this!!"
Expect your poor Mom to waver back and forth, but have those "I am your accountant; let's have lunch meetings". Got so my bro wasn't even interested in the subject. Had his pre punched sheet to put in his notebook every Month and was quite happy to be rid of thinking of bills and etc.

Sister's pretty smart here -- just stay away for a bit. If your sister knows you well enough to know you aren't going to steal from her and you two are on the same page, then just give Mom a little breathing space.

Don't discuss Mom's finances with her any longer -- just take care of them. A lot of times the things that get a dementia patient upset is someone else bringing up a topic as though life is still normal. Handling their finances is scary when their brain isn't working well, so they naturally get fearful that someone else is handling them. Just don't bring it up, and that topic will likely fall by the wayside.