I have very fine, thick hair. It tangles very easily. When I was your sd's age, I had that short bowl haircut, because my mother didn't want to deal with it.
Anyways, I think you can do a couple of things to help. Take her to get it trimmed and add some long layers in (ala the Rachel haircut). Have them thin it out, as well. This will help combing it. Also, it's time to switch out the kid's shampoo. I believe it is damaging her hair and making the tangles worse. I would switch to something like Pantene or Garnier or VO5, something super rich. I also think for a girl this age, she doesn't need to wash her hair so much. Once a week at most should she shampoo. The rest of the time you can just condition it or wet down. If she wants to keep her hair long, after her baths and her hair is combed through, I would braid it. And, keep it up or braided as much as possible on play days.
I don't know if you have seen the infomercials for Wen Haircare. But, it's a totally different way to cleanse your hair. This kind of technique will really help her hair and comb it without much less pain. Instead of shampooing with a regular shampoo, use a conditioner to cleanse. Most conditioners have very mild cleansers as a base and can clean the hair and scalp just fine are are not abrassive to the hair. I looked into switching, myself, but could not afford Wen. After some research, I discovere that I could get the same results using VO5 conditioners.
You apply a generous amount to cover the hair and massage it in really well, massaging your scalp. Then, let it sit while you finishing washing. Then, comb through with a wide-toothed comb and rinse thoroughly. You don't need to use anything else. I used to have frizzy, hard to comb, DRY hair. I have been shampoo free for 12 months and my hair is shiny, soft and looks the best it has ever looked. My frizzy hair is now naturally wavy, when I don't use a straightener. I also use this technique on my kids with great results. My daughter has very fine, tangley hair, too, but it is much easier to comb since I switched to being shampoo free.
Sorry to write a book. But, I hope you can find something to help your stepdaughter!