Hello there - sorry things are in turmoil just now. Do you mind saying how old your DSD is - someone said about 11 yrs. I think children in this position often get scared when the sw arrives as they don't know what's coming, new plans maybe, or worried about what they will be asked. Does the sw talk to DSD on her own, or with you present. SWs are meant to see children on their own but I it's always difficult to do this, and sometimes just makes the child more anxious.
I'm not at all surprised by what the SW told you about DM - I have met so many women like this, and can never understand it, but I know you said the SD controls the DM so it may be that she is just too scared to leave him. Women in this situation seem to get "paralysed" with fear and of course the blokes have usually worn the women down over many years, so their self esteem is rock bottom which makes it harder to take the right course of action. Not making excuses for her, it's just that I've seen this so many times.
Am trying to figure out in my mind the legalities of this situation. All children in care need to have a "care plan" and this can change overtime, but by the time of the final hearing in court, there has to be a definitive "care plan" so that the Judge can see how the LA intend to secure the future of the children. Do you know if the LA are going to request Care Orders be made on all of the children in the family, including DSD? If not, maybe you can ask. I know it seems strange to request a Care Order when DSD has her dad with PR but I'm not sure how else they would remove PR from her mother, but there may be a way.
From what you've said I don't think there's any chance that the care plan for DSD will be for her to return to her DM and SD - none at all. I know it's easy for me to say that but this DM has had so many chances of help etc that the LA will have the evidence they need to request the Judge makes Care Orders on the children. They may be criticised for not removing the children sooner, as I have known this happen a fair few times in court.
I think you are doing exactly the right thing by not asking DSD anything and you're right she will maybe talk in her own time - often it's a bit like a dripping tap, it comes out a bit at a time. If she does say anything, I'm sure you know you need to record it, and your reaction is important too - you need to be matter of fact and not appear upset or shocked, just calmly say something like "that must have been scary for you" - acknowledging how she might have felt, but not in a way that suggests you want her to say more at that particular time. Often children find it easier to talk if you're doing an activity together - drawing or doing something crafty maybe, or even in the car. I often used to drive children around as they seemed to be able to talk more easily in the car, no direct eye contact needed. She is almost certainly feeling confused herself and maybe even a bit guilty about leaving her mother, or worrying about whether she's ok as SD may well have been violent to her.
Do you notice anything before or after contact, as there's often a reaction of some sort. I think you said she gets to see the other sibs and her GPs and this is important - I get the impression she is the eldest but obviously don't know. I don't think counselling is the right thing for DSD especially at the point in time, and probably not till she's much older. It's hard for children to have a stranger asking them Qs and expecting them to be able to talk about their lives - hard enough for adults. For younger children I think play therapy is a good idea but very expensive so LA won't usually pay for this.
Don't under estimate what you and DP are doing for DSD - she doesn't need counselling - she needs what you are giving her - stability, a safe home where she is loved and no pressure put on her to talk about her life. The other thing is of course that things will drag on for several months probably before the final court hearing, especially as there is a large sib group, and this makes it all the worse. Can you ask the sw about the care plan and maybe this will help put your minds at rest a little, although of course the Judge makes the final decision so sws can't say what will happen, but Judges almost always follow the recommendations of the sws and other professionals involved in the case.