My H had an A, B, C sibling set up except he was the favoured firstborn, so had the status of B in Op's situation. His 1 year younger brother had the status of A. Both parents hit both kids, proper beatings with a dog leash or wooden spoon, but H's brother by far got the worst of it because he was more "difficult".
Their sister arrived 5 years after my H and was definitely a C. She witnessed the beatings and was mocked relentlessly - the mockery was relentless, stemming from FIL, who wielded it against his wife and kids. MIL also engaged in it against the kids, and H engaged it against his siblings. A trickle down of humiliation disguised as jokes.
By the time I came on the scene, when H was on his 30s, MIL and FIL had obvioudly stopped the beatings (kids too big and could fight back), and the mockery was more watered down. And they were putting up a pretty convincing happy family appearance, which the kids played along with quite devotedly (it sure fooled me). The kids and MIL were all still in thrall to FIL. But I noticed how distant the kids were with each other. They were not friends, did not consider each other as friends, had little to do with each other outside family get together. It was... odd.
But BIL clearly HATED his mother, even though it was FIL who mostly beat him. The resentment was palpable, and sometimes burst out at family get togethers. MIL found this upsetting and talked about it with H, me, and SIL. She seemed to regret how she and FIL had parented the kids.
At one point, we encouraged MIL to talk to BIL. They sat in his car and he told her exactly what he thought, how he had experienced his childhood, and how he felt then and now.
MIL was shattered and crying. But the next day, she told him, and then us that BIL was "exaggerating, we weren't that bad, we were loving parents who made a few mistakes" and BIL should "forgive and forget instead of walking about with all this excessive anger".
This is how dysfunctional families work. The dysfunction comes from the parents, who make all of the children feel unsafe, which makes them competitive for parental approval (because it protects them from beatings and mockery), which terminally poisons the relationships between the children. This poison resonates on and on into adulthood.
From what I've seen in my IL family, it's almost impossible to overcome this. The divisions go really deep, and the historical pain and injustice just never gets addressed and the family remains this dysfunctional mess, where the poison erupts every now and again in a violent conflagration that everyone then rugsweeps.
And it starts to affect the next generation as well, the envy and competitiveness and currying for Gramp's or Granny's favour starts to emerge in the cousins.
It was really bad in my IL family and I withdrew myself and limited contact between my kids and the ILs because of it.
So all I can say to you, OP, is that A is justified in her pain and anger and desire for her siblings and parents to acknowledge how horribly she was treated. But it's unlikely that that acknowledgement will ever come. All A can do is accept this, decide how much contact with her abusers and their sibling collaborators she can tolerate, try to heal with therapy, and live a full rich life with her FOO in either her periphery or in her rear view mirror.