Am I wrong to feel I need to let go of my Mom for good?

During a trip down south with my mother & brother to Nashville; my daughter & her fiancee' drove hrs to visit us for a couple days from KY. We hadn't seen her in over a year and a half. I was excited to say the least; taken my Mom to a place she's always wanted to go; the Opry & Graceland! My problem started when my 44 yo half-brother that has down's syndrome forgot to brush his teeth. He jumped out of bed not wanting to forget this ritual. He went to the main bathroom in the hotel where my daughter happened to be. My mother jumped out of bed, yelling at him to the bathroom and once there was pulling his arm & tryin to tell him his toothbrush is in the bedroom bathroom. She apparently moved their toothbrushes to the bedroom bath but didn't tell him that. Anyway, my step-brother; being very rigid and bound to his structured behavior; stood there at the sink looking for his toothbrush. My daughter saw my mother slap him on the back when he wouldn't move to the bedroom! I heard her say from the bathroom, "Grammy, you didn't have to hit him!" OMG, I was stunned & mad! I thought, if I get up; I'm goin to make things worse, so I will approach her in the morning about this. I also have alot of faith in my 23 y/o daughter's judgement. I thought; she said the right thing at the time. I laid in bed until I heard my mother snoring; then got up and asked my daughter about it. She said my Mother after yelling, pullin on his arm; she then "wailed" him on the back with a big slap! She had the look of evil in her face she's never seen. Her fiancee' told her he heard the slap in the livingroom! I cried all nite, and finally fell asleep. I have been my brother & my mother's caretakers for years; spending 3 nites a week there for over a year. She's been afraid to stay alone now that she's 90. I get zero help from my other brother's who live closer than I. My problem is, after confronting her calmly the next morning alone, she began calling us all liars! She first said her hands couldn't hurt him with all the arthritis. Then she says she "tapped him" on the back. Now she tells people she only touched him on the shoulder! I heard my daughter tell her she didn't have to hit him but still I'm a liar & so is everyone else! My daughter would never lie about her grandmother hitting her special needs son & her fiancee' has no motive to lie either. She never would have said that if it was just a tap. It's ridiculous! She has hit us kids & althought I've never seen her hit her son before; I have seen her emotionally abuse him when he doesn't come back with something she told him to get. She gets this evil, uncontrollable angry look to her. She is very narcissitic, which I've learned after reading many other stories on this site. Anyway, the following day I told her how my daughter would be hurt & never speak to her again for calling her a liar; and she said, "no reat loss!" What? Then she proceeds to say, what has she done for me? I said love you like a granddaughter should! She said, she didn't even send me a card for my birthday. I told her her own sons didn't send her a card! I'm hurt my mother not only denies hitting her special needs son, but says these hurtful things that a grandmother shouldn't say. I have stuck by her thru her own life supporting her through a bad marriage that she destroyed, her health, her emotionally well-being everything! I am my brother's guardian and told her if I ever see or hear of this again; I will take her son home with me. Her response, to that was "not for long!". I just wish she would have acknowledged it by saying I shouldn't have done that; I lost my temper, something! Instead, she denies it, called us all liars and said hateful things regarding my daughter. My neice, who've I been very close to; is not talking to me. She had called during this time & my mother told her about it and that it never happened, she's a Christian woman, blah, blah. I spoke to her & told her what my daughter told me and gave the phone back to my Mom. Get this, after my Mom got off the phone, she looked at me and said, "Lynn doesn't believe you, we're like this...crossing her fingers together! That was like the evilest thing she could ever say; while she lied through her teeth! My problem is how do I see my brother without seeing my Mom? I am his guardian, and will still take him when she is gone. I feel like I should report this; but I didn't 'SEE" it happen. I believe my daughter felt the need to say it because she did hit him and her fiancee' heard it. She is totally losing it now and being downright mean bad mouthing me and my family for really, doing the right thing and confronting her. This has made me realize she is toxic, hateful and a user who will turn on you the minute you go against what she says or does. She's done this to me so many times before. My husband & children don't want me to go back cause she's hurt me so much over the years. Am I wrong in wanting to let go of her forever over this?