Lots of older siblings end up watching younger siblings in addition to or in place of adult care. In general, how old do you think a child needs to be to be completely responsible for another child? Does the age of the younger child matter? Did you watch/get watched by siblings when you were growing up?
I really feel that it's less a case of age and more of emotional and mental maturity on the part of the child (as well as the parent entrusting them) and the environment in which the babysitting will take place. Knowing only what I know of the characters thus far, I think I'd be more comfortable with Mennie watching children in my home than the horny knucklehead her mother has hired to watch her. I'd say that throughout this storyline Mennie has proven herself more capable of taking care of another, in regards to the intelligence in and the gravitas of (HA! Keifer Sutherland, I used that word in a conversation! In your face! ) understanding the responsibility given to her. Has she made all of the best choices? Maybe, maybe not, but given her circumstance she seems to have made clear and thoughtful decisions based around the elements she has had to work with.
As for me, I am the oldest of seven so it goes without saying that I had duties of watching siblings from time to time. I was also a very precocious and responsible child so I even had parents of friends instructing me to be the good decision maker when there'd be little to no adult supervision. Unless a parent has made an effort to lock their children to their hip, I believe that situation always comes up with siblings, varying from degrees of actual babysitting to the simple wish of, "watch out for your brother/sister and hold their hand when you cross the street," when they go out to play together.
I wasn't ever asked to watch my sisters without adults at least being home until I was in my teens, but there'd be momentary baby-sitting stints for me I suppose, like if Mom wanted to go hit the restroom or take a nap or something. She didn't leave us home alone with each other until we were much older, and even then I seem to recall there were rules about climbing and cooking in the absence of an adult.
The concept of babysitting didn't really exist where I lived. Me and my two younger siblings were left alone even before I entered school - just as all other children. I was only a little older than them (2 and 5.5 years) and was not told to watch over them.
When I was 4 or 5 and my sister was 2 or 3 I was left outside of a store with her in the pram. Unfortunately I rocked it so hard that it fell over and she fell out. Oopsie! But she wasn't hurt. Adults near us helped and put her back in. Nobody thought anything about it. Everybody left prams with babies in front of supermarkets and shops, with or without siblings. As a kindergarten and primary school child I often stood outside supermarkets rocking prams with crying children whose parents I didn't know. It was not unusual.
There was one incident that made the news. A mother had left her baby in front of a supermarket and when she came back it was gone, with the pram. Panic. Police called. Turned out: the grandmother of the baby had come by and taken the baby with her. Silly old woman, to scare the poor mother like this.
The stones at the top of the roof of the house at the corner of my street were loose. Sometimes some fell down. There was a shop at that corner, I think a bakery. One day a mother parked the pram in front of it. Took the baby inside with her. When she came back, a stone had fallen right through the pram. She was so glad she had taken her baby inside that day.
I was in England with a host family for a week when I was 13. There was something in the news about the parents neglecting her daughter by leaving her home alone without a babysitter. Confused me like hell.
Nobody thought anything about it. Everybody left prams with babies in front of supermarkets and shops, with or without siblings. As a kindergarten and primary school child I often stood outside supermarkets rocking prams with crying children whose parents I didn't know. It was not unusual.
Are you from Iceland? I saw something once where in Iceland they have places where you park strollers/carriages/prams with the baby still in them outside of shops, like a bike rack but for infants.