Razzle, this isn't a usual situation.
A child was adopted into the family, a plan would of been agreed upon. None if which existed in the same way in your GP's day.
From what it sounds like, there are still problems, as said, could be physical, could be EA, the child may have a CIN put in place on entering school.
What I will say is when people tended to have 26 GC, no-one took on board emotional needs. Life has changed dramatically
This is a damaged 4 year old compared to a 7 month old baby. The 4 year old needs consistency, the GP's are giving that to both, it's just one gets more than the other, probably because that is what's needed.
This really is about the Parents favouring the Sis, now the children are being dragged in.
It should of been sorted before they came along.
Whatever is decided, this adopted 4 year old shouldn't suffer.
In two years time, when the OP's child is old enough to be aware of stuff, things will be different. The four year old will have a diagnosis and new plan.
Thankfully there are families like the GP's, who don't drop the adopted child when a blood related baby comes along.
30% of adoptions fail completely, a higher number of both adoptive and long term foster placement don't work.