I cannot offer any insight to almost anything except the sexual abuse.
I have ptsd from multiplke counts o sexual abuse, and all I can say is, I experience every incident many, many times through flash backs. When I get a flashback, which can be caused by a smell, a sound, a similar scene like somebody doing something a person did before or after the incident, or even by being touched in a certaina rea where I was kicked or hit I am thrown back into the very moment.
A flash back isn't a memory. It is the exact same thing ahhpening. All. Over. Again. Every sensation is the same and just as real.
When I fall into a flash back, what helps me most is to be squeezed. A very strong, warm hug by my SO, or holding something with a very distinct feeling to focus on or a very distinct smell.
It is hard to recognize the flash back as myself not being in that very moment again, so something very stimulating in the real moment helps pull me out.

I also have ptsd from abuse from my parents. Again, the flash backs can be caused by what I listed above and are also just as real. Again, the same things help pull me out of the experience.

After having a flash back I am physically, mentally and emotionally exhausted and sometimes I don't have the strength to stand on my own. In this moment it helps to have something simple to focus on as I get my strength and breath back.

Something I highly suggest, if she has flash backs and what has worked for me, is a plushy or fuzzy toy that has been sprayed with a strong fun scent like orange or lavender. I would use my plushy scented toy whenever I was relaxing so I didn't have the toy and it's scent only associated with flash backs.

When my SO has a flash backs (he has ptsd from medical malpractice and childhood abuse) I stroke his hand and flick some cold water in his face. The water thing could just cause a panick in lots of people, I know, but this is what works for him.

PTSD is something that a person can opvercome with the proper tools and skills and support. I am not assuming your post is your neice having a flash back, but I want to help as best I can. I hope I ws able to help or shed some light. Feel free to ask me questions.