in theory she can just slip in with us
I think this is very, very unlikely. There is a good chance that she will need a high level of care throughout her life, and she may never be able to live independently. So, either a SAHP or experienced nanny. Special school and lots of appointments.
Are you willing to give up your job, or go PT in a job that allows lots of flexibility and time off? Because even if you have a nanny, even regular appointments can put your job on the line.
I adopted my DD9 from a background of severe neglect, including an unknown amount of alcohol abuse while pregnant.
She is amazingly resilient, compassionate, and has many talents, and I have every reason to believe will be able to live independently (perhaps a bit later than her peers), find a job that is satisfying and pays the bills, and has a range of positive relationships. She is sociable and copes with mainstream clubs and childcare, and is low-average in mainstream school.
BUT! She has a range of not-quite-clinical-level difficulties. Executive functioning, emotional self-regulation, speech and language difficulties, anxiety, she is impulsive and finds it difficult to concentrate, a little bit of attachment/control issues. When you put them all together that means an endless round of assessments, therapeutic input of various sorts which usually results in exercises to do at home. Endless advocating for her because in general teachers etc are happy to say 'that's terrible' when I outline her past experiences, but cannot see the connection between her experiences and her behaviour via the long term neurological and psychological damage.
I now work PT in a basic admin job, having previously been in senior management, post-grad degree, career with travel etc.
I don't regret it for a moment - and if the FAS is so obvious in your sister, then my situation is probably best case scenario. And I don't have any other children to be affected.
You need to get VERY assertive with SS right now. Because they will run walk away if at all possible. They need to be doing a full assessment of need, and a package of support to meet those needs. And written commitment to continue to provide as necessary.
Otherwise it may be best for your sister, as well as the rest of you, for her to be in long-term foster care or adopted. In either case it is very likely you would still be able to have contact and maintain a relationship. Possibly you will be of more benefit to her not having the day to day responsibility of her care, but saving your energy to provide love and advocacy.
PM if you'd like to chat.