@Namechange543 For me it was a question of coming to terms with things as an older child, processing, moving on. I have had no mental health issues at all as an adult, career, my own family, etc, so take hope. What happens from now is important. Very difficult childhood experiences are in fact very common. I really do feel that it is not so much what happens to you, more how you deal with it, is important, and far more is known now about how to help children with traumatic experiences compared to when I was young.
Re the sort of psychological help available nowadays I will attach links below.
From your post to me replying to your questions (which you could get deleted, just that post?) my personal views based on what my family experienced is that the ideal (nb see comments on "ideal" below!) thing would be for them to be split up while having therapeutic work and then reunited, as long as they can go to individual foster carers who are experienced, mature, emotionally intelligent, understand the dynamics, will facilitate contact with you and other members of your family, seek really good help - see links.
I do think you need to put the needs of your children first in relation to what you decide about one or more living with you now or in the future, but even if you do decide you cannot at the moment, it is not a never say never situation, if things go well. And just having people who have their backs, like you, in their lives will make such a difference. If you were able to push for the right psychologicalhelp and regularcontact with each other and you and your family, which is well managed, you would be helping them a great deal.
However... life is not ideal and there are record numbers of children going into care, and much will depend on the individual social workers and foster carers involved and I would really strongly advise you find a really good family solicitor in the relevant LA who know the inner workings of the LA so that your input always gets through to the right people and you know how best to get things done. The last thing you'd want is for them to be lost in the care system, as it could be horrific for them and they could face more abuse. I don't have any experience of this as an adult but I am aware from threads on MN that having a shit hot supportive solicitor who knows local practice and the best way to communicate with the LA will make a huge difference. I would get that asap, start trying to find someone Monday. NB not all solicitors will be born equal or be shit hot... you need recommendations ideally and not from the social workers ...
These are some resources which may give you insights and so that you can see the sort of therapeutic help available - this first video is narrated by a child but it is in its entirety a really helpful summary about the ideal approach taken to help a child heal in these circumstances:
This is the website of the organisation who produced the above video it will have lots of useful info about the sort of help available:
beaconhouse.org.uk/?section=welcome-to-beacon-house
This is a video about where they help an adult, might be relevant for your sister:
This is an organisation who helps "birth" families who may have really good ideas and insights about what you could do - again no personal experience but have seen them recommended on mumsnet:
www.frg.org.uk/
Please excuse all the mistakes, and length, had to bash this out quickly.