Did DS really work harder or was he just lucky to be more academically able? Is it possible dd has a still unrecognised mild learning difficulty?
We'd need to know much more really.
I come from an extremely dysfunctional family. Sis is mum's gc I'm her sg, I was dads (with ulterior motives on his part) with sis as sg and bro was basically completely ignored!
My sister can do nothing right as far as my fathers concerned. She could become a top class neurosurgeon, devoting her spare time to charity and an excellent mum and he'd still find something to criticise. With mum though she can do no wrong - she's in her 40's a single parent, has never held down a relationship or a job for more than 5 years, constantly over spends, spoils her kids materially while also basically expecting them to fit round her life, betrays friendships, relationships and employers, lies constantly, and has a substance addiction issue. Mum bails her out every time she gets in serious debt inc repeatedly being evicted.
I can do no right in my mothers eyes. My ex-husband cheating was due to my not being a good enough wife apparently, my being unemployed due to ill health is laziness and setting a poor example for dd, dd wears too much make up and has too much freedom. I'm too critical and argumentative and should be more passive and quiet according to her.
This from a woman who has stayed in an abusive marriage almost 50 years "your father would never have cheated on me I'd never gave him cause to" more like nobody else would have him!!
Brother is a total adrenaline junky mainly as being a daredevil was the only time either parent even noticed him!
Being a golden child comes with its own baggage and pressure. In my case it was csa, in my sisters I wonder sometimes if she's trying to push it so far to see IF there's a point at which mum will go "hey! Enough!" Because if there isn't, is there any reality to mum's supposed love for her? Being a gc isn't always a blessing.
I also believe I'm mum's sg because as far as she's concerned I'm to blame for her marrying dad and ending up in the abusive marriage (shotgun wedding), whereas sister I believe was unplanned too, that my parents hadn't planned on any more after bro (definitely planned) was born. That dad resents her as an extra burden financially. Parents are Catholic so abortion wouldn't have been considered an option.
The kind of examples you're offering are almost exactly what my parents do of how they DON'T favour anyone -
All had similar size bedrooms
Equally supported academically (patently not true)
All equally treated in the wills
Same amount spent on each child (to the penny) at birthdays/Xmas
But -
sis was allowed more choice on bedroom decor, dad chooses poorly gift wise for sis, mum gets me gifts sis would like but are meaningless for me, bro's gifts are bought last min with little thought.
Mum bailing sis out to the tune of £10,000's will obviously affect whatever inheritance might be left over (I'm not interested in inheriting from them - that's caused arguments too, but I just find it a joke that they claim they'll be leaving us all an equal inheritance but sis has already had I believe close to £100k in bailouts)
Mum is much closer to and babysits/spends much more time with sis' children.
Important news is told to sis first, then me sometimes months later and bro is lucky if my parents tell him at all, it's usually me makes sure he knows. This inc serious illness/bereavement of relatives.
I've met/read people who claim a gc/sg setup and it's just the usual sibling rivalry but I've also met/read people who really are dealing with this dynamic and struggling.