Multigenerational Trauma

As it's summer, Penelope is staying with her grandparents again. Each year gets better and better as she gets older, gets farther in her healing, and takes more accountability in her life. Her RAD and PTSD will probably never allow for a smooth summer but the other challenge is my mother.
Unlike how I approach Penelope, with learning opportunities and faith that she will continue to grow and heal, Grandma is a lesson for me in patience and tolerance and evaluating if my actions will result in a positive outcome. In other words, I pick my battles and try to minimize the chaos the best I can. It is just really hard when it involves your child.
Last Tuesday, my mother had a dentist appointment late morning. My father had committed to my twin brother B, to drive him to a post surgical doctor's appointment. If Penelope was left home, she would have been home alone about 2 hours. She has done that many times this past year, no problem. But my mother insisted she go with her when it was time to leave.
You may think "What's the big deal? She should go." I'd agree under normal circumstances. But my mother insisted, telling Penelope she didn't trust her to stay home. She would take her to lunch afterward. Now if you know my Mom, this is really about her wanting to have someone to go out to eat with. She loves to do it and jumps at the chance whenever my Dad is preoccupied like that day. (He likes to keep a tight reign on the purse strings.)
You might still be saying, "It's still not a big deal. So she is putting Penelope down, saying she can't be trusted, as an excuse to make her eat lunch with her. Penelope gets a nice lunch out for her troubles." Again, I agree. Not a big fan of the apparent lie/put-down but could still tolerate it. But again, this is not a normal circumstance.
My mother's dental hygienist is my sister-in-law, the mother of Penelope's cousins she sexually abused. She didn't tell Penelope that was where they were going until they were halfway there. And my SIL didn't know either.
It's been 3 1/2 years since the incident and Penelope has been all but erased from their family. When the kids all about her, the conversation is redirected. My SIL has forgiven her in her heart, she knows of Penelope's past and mental health issues, but can't let her around her family. My brother B carries even more burden because he was in charge that night, and still has a ways before he will ever forgive her. The kids have not shown any signs of trauma other than missing their older cousin. Holidays and family gathering are always seperate or without Penelope and I. Well, I'm invited but how does that happen? Penelope is far from the same person and carries a ton of guilt on the issue.
With those facts, one would think that my mother would know better than to take Penelope to the dentist with her. There are two versions of what transpired in the car.
Penelope's version: Grandma told her where they were going. Penelope told Grandma that she can't go there. "I can't!"and became very upset. Penelope told her how upsetting it was for her to even go to their house with Papaw to let the dogs or when they were 5 states away on vacation. Grandma told her that they are almost there and she can either go in and sit in the lobby or stay in the car. Not to worry, her Aunt won't see her if she sits in the back of the lobby. Penelope choose to stay in the car and called me at work crying. She felt totally abandoned and unheard. She said grandma said "I've got dark secrets too, and I've learned to deal with them. You need to learn to deal with yours too."
Grandma's version: Penelope never said anything about having a problem. She didn't decide she was going to sit in the car until after they got there. ... That was the version on the day it happened after she learned Penelope called me and that I was going to call her to discuss. She had answered the phone "I don't want to talk to you." So I made a point to not talk too much about it, especially after getting those answers which I felt were complete fabrications. She was sticking to the reason for taking her at all was because she couldn't leave her at home, she didn't know how long she was going to be gone even though she didn't have any errands. But a soon as I realized they were in a restaurant, I knew the real reason why Penelope had to come along. It doesn't help that while Penelope was sitting in the car, my Dad had to drive B there to pick up his wallet from his wife. Penelope, based on my Mom's wishes, could have been sitting in the lobby when B walked in! Penelope would have flipped....out! I don't think B would have done anything except maybe walked out but not one of them needs to be put in that situation. B did end up seeing her sitting in the car on the way out. Penelope didn't know until my Mom told her after the fact. When Penelope told her grandmother she can't see Uncle B, it would be too upsetting, my mother told her "Well, he is my soon so I'm going to see him when I want" referring to the fact she got to see him when he picked up his wallet. Her argument doesn't make sense!
So the issue came back up this past Saturday and she had forgotten her original excuse of not trusting Penelope to stay home by herself and said it never occurred to her that it would be a problem. And if Penelope had said something like "Pull over, I'm getting out" she wouldn't have made her go. I told her that Penelope shouldn't have to go there before she is heard! She tried to claim again that Penelope didn't say anything until they were there and I said, "So you are saying Penelope never said "I can't go there, or explained how hard it was to be at their house with Papaw?" Her response was, "I don't remember, it's been a few days..she might have said something like that but I don't remember..." Ugh!!!!!!!! She continued to tell me what she had told Penelope about her dark secrets. Penelope had told me the dark secret my mother shared and hearing it, made me angry because in my opinion it's nothing compared to what Penelope did. Without going into details, my mother had said something hurtful about her own mother in earshot of her while she was laying in bed dying of cancer. She was a little younger than Penelope. Her Mom died a year or so later. My mother was near tears when she was tip-toing around it internally debating if she would share the details with me. But I already knew from Penelope telling me. I said "If it was something I had done to you, would you have forgiven me. She paused for a second (to put herself in her mother's shoes) and said she would, but didn't know if her Mom ever did. I asked her if she felt like her Mom want a forgiving person? She said she didn't know. The shame in her voice, you could tell how raw and real her self-hate is for what she did. It blew my mind because it was such a forgivable act. To have carried such a burden for over 50 years and feel like such a horrible person for it explains a great deal about her. I just can't imagine a mother not forgiving their child for it. She was a child, grieving her mother's illness, not understanding in a rational way what was going on. I told my mother I feel like she is holding on to the emotions of the child and not processing it as an adult. It's like remembering a scary dream and still feeling scared when you know rationally it's not real.
I know there are other traumas in my mother's childhood that she doesn't talk about that happened to her and to think thus is how she feels on the smaller things makes me feel very sad for her.