My parents had two girls me (17f) and my sister (14f). I have always felt like my parents saw me as a babysitter, as a third adult in the house, and have expected me to be a 40 year old kid/teenager.
Whereas my sister was their baby. They spoil her rotten. They adore her. She has never wanted for anything and they do everything to make her happy.
I was expected to help take care of my sister for as long as I can remember. She's not special needs, was never sick, wasn't even a preemie either. But that was the dynamic that was set up. I remember I would come home from school and mom would have me help feed my sister.
She would have me watch her then while she was in and out of the house. I would be told I needed to play with her if my sister was upset. She'd miss me a lot and my parents would insist I dedicate my time to her once I was home.