@Milgram, I’m a little bit terrified to be responding on AIBU but I wanted to raise my head above the parapet to help you (and your sister). I’m a single adoptive parent of two non-birth related children, a teenager and a five-year-old (just to make life extra hard!)
I honestly think you would get some insightful, wise and considered advice on the Adoption board here and I would heartily recommend AdoptionUK, although the forums are much quieter than they used to be, for a myriad of complicated reasons. There are also amazing closed groups (Sarah Naish’s Therapeutic Parenting on Facebook, for example).
Your sister and husband will undergo rigorous ‘training’ and assessment by social workers; but, honestly, NO training in the world prepares you for life 24/7 with trauma-experienced children. You will know that if you work in PRUs. And you, presumably, are able to go home at 3.30pm?
I have heard adoptive parenting described as parenting-plus to the power of about a million. Like foster carers, we are the proverbial swans protecting and covering for our little ones while paddling at about five million miles an hour below the surface just to try to keep up with our peers and the rest of society and put on a good show.
It WILL undoubtedly be difficult (there’s some research - though not a massive study, which always sets me twitching - that reckons that two-thirds of adoptive families will experience significant or some difficulties).
The majority of babies or children who come into the care system nowadays are removed from their birth families for reasons like domestic violence, drug and or alcohol addiction, neglect, physical and or sexual abuse - and often a combination of more than one of these (or the risk of future harm, if removed at birth). This obviously leaves a legacy and no amount of back-filling of love and attention can undo the damage that is caused physically and psychologically to our children. It would be good for your sister and her husband to read up on Foetal Alcohol Syndrome and Neonatal Abstinence Syndrome, as well as attachment disordered behaviours.
My own DD1 has a significant disability as a result of her in utero experiences - but, against all the odds (and there were many) - she earned herself a place at a super-selective grammar school, where, with incredible pastoral support, she is thriving academically and now has a (few) good friends. Honestly, one friend = we’re winning at life! DD1 spent the first week of her life in SCBU withdrawing from opiates and had two foster placements before she was a month old, so even though I’ve had the privilege of being her mum for 13 years (it’s our ‘anniversary’ this month), her attachment to me is always ‘wobbly’ and her consequent behaviours can be challenging (stealing, lying, extreme anger, aggression and violence on an almost daily basis). On the plus side, her school SENCO and form tutor and head of year, tell me she is possibly one of the politest and most respectful young people in her year group (school is challenging as a disabled and adopted kidso she saves up all her angst and frustration till she arrives home and erupts like Etna!)
It’s a good job the five-year-old is an easy-peasy, happy-go-lucky, gorgeously larger-than-life socially easy butterfly (for now!) as I think I would be resorting to gin on my cornflakes rather than milk - and I really hate gin...