I was the eldest of 2, and my DB was exactly 2 years my junior. My DM expected me to watch him, to play with him etc. He was the Golden child and a badly behaved little boy. I would get the blame for anything he did, as I "should have known better".
Later when we were left chores to do he would just disappear off to his bedroom and I would get it in the neck for being lazy
. I felt very much as a child/teen/young adult that 2 children is the wrong number. If one is the favourite then the other just isn't. Stupidly, my DM was the eldest of 2 and felt exactly the same about HER younger brother, yet still did it to me.
DB went off the rails bigtime in his late teens and DF expected me to sort it out. I really wished there had been another sibling to shoulder the burden. Similarly once my DM was widowed it's me she looks to to look after her. DB can't be bothered.
Because of my upbringing I said I'd have either one or three, not two. Ended up with five. Four within 5 years, then a late baby 15 years later. Not once did I ever expect any of them to look after their siblings. I was very conscious of not putting demands on the (then) only girl.
We couldn't afford daycare so we worked around them in shifts. Me and DH passed on the stairs occasionally, but there was always at least one parent available. FIL retired when DC3 was at playgroup and took a big part in looking after the younger ones because he asked to, not because we demanded it. He felt he missed out on his own DC.
The older ones (late 20s/early 30s) reminisce when they get together about how hands-on their dad was.They all keep in touch with each other independently of us and will meetup and go out when they can, so I don't think we can have done that bad a job.
Unfortunately they aren't tolerant of the youngest and she's ended up a billy-no-mates. I'm hoping they'll connect with her more when she's an adult.