I think that's the point I was trying to make NYD - the way we were cared for (or not) as children has a huge influence on the adult people that we become, and so judgements shouldn't be made because it's not possible to tell life stories on here. YOu don't sound like you are making excuses.
It's interesting that you say her emotional age is around 6/7 mark and her chronological age is almost 12. There is very often this kind of gap in children who have had troubled pasts, and I think it is important to allow her to regress sometimes (maybe playing board games aimed at younger children) and drawing and colouring books for younger children. I think you will have to keep this emotional age in mind when you start talking to her about her past, and go very slowly.
Another idea is to draw a tree with the long roots showing, and they can represent the child's background, and talk about past life , and then the branches and leaves are her present life. Tbh even just getting her to draw pictures of B/SIL and baby and her and then you and her dad and your children and her, and maybe pictures of her mom and her and whoever else she puts in the picture, and foster parents if she had any. Sorry I don't know background but you will of course. Sometimes these drawings can be very revealing, even just by noting where she draws herself in relation to other people in her life. One child I worked with drew his dad then him and his brother and then his mom and coming from the dad to the mom were big bold lines some of them zig zag and he volunteered the information "that's my dad shouting at my mom, and then he drew a picture of his mom with tears coming down her face and said "that's my mom being scared of my dad" - it accorded exactly with real life because we knew his dad was a very controlling person and his mom was too scared to stand up to him.
Thinking about it, it might be best to just draw together first (you all need to be drawing too) not just watching her, and be interested in her drawings and get her to talk about the picture if you can, but don't press her. You and her dad and your kids could talk about their pictures, so she feels more comfortable.
You can then get her to draw all the houses she has lived in and put smiley faces/sad faces/angry faces etc wherever she wants to. It would be useful to have some sticky faces with different expressions, to make it a bit more fun.
I do have other ideas, but I'm sure I've said enough for the time being.
Would be interested in how you get on.