My mother did exactly the same
I'm the eldest and only girl,another brother (no2) and twins (3&4)
No3 could never do anything wrong growing up (he's now a narc druggie who beats her up if she doesn't give him money/lifts/anything he wants) and she put up with my other brothers,she made me the scapegoat
She played us off each other all the time growing up
As soon as I had kids myself,it was my dd who couldnt do anything wrong and my son couldn't do anything right
They grew up a bit and suddenly son couldnt do anything wrong and dd couldn't do anything right
Then my no 2 bro and his wife had a baby and that child was everything
She'd twitter on about my nephew while ignoring my dc (I remember sitting there while she was banging on about how nephew had learnt to ride a bike while ignoring my dds exam results and how much money ds had madde for charity)
Then suddenly it was my ds that was back in favour and dd wouldn't do anything right
I saw it all happening and knew what was going on and she knew it so I was pushed out in fear of revealing it (I'm nc)
She lives in fear of me as she knows I can see the bare naked truth of her and she can't bear it
Writing this,I know it doesn't make sense unless you've lived it and know how they work