Fascinating thread. My DH is the scapegoat, my SIL the golden child. I would not say that the PIL were narcissistic, but we think DH was a more difficult baby apparently, they failed to cope with the transition from having one child to having two children, and their marriage was in a different, more precarious place when he came along. That all crystallised into DH being the problem. I also think they had a very set attitude to gender roles and preferred parenting a girl, though FIL was a good father to DH in many ways (when they went off to do 'blokey' things, not when in the family group).
As so many have said, the damage to each child is very different. My DH is independent, married, a parent, has a few key very long-standing friendships and despite quite a bit of residual emotional damage, is pretty sorted. SIL is unable to sustain personal or professional relationships except with slaves/doormats, and is more and more alone. She has never attained any proper adult distance from PIL, is narcissistic, aggressive and massively entitled. Underneath it all she is insecure and troubled. It is so desperately sad.