And no offence to you, but this sort of thing has been considered standard for many, many years in many, many countries (for example, I just saw a documentary about Russia with an instance of such). The oldest daughter becomes mother to her siblings, because it is indeed forced by circumstance. If she doesn't help, then The Household Falls Apart and everything goes pear-shaped.
^^^^ This.
My family has the same kind of history. When mom is perpetually pregnant, it takes a toll on the body. The eldest children are crucial to keeping the household running. In the case of my maternal grandfather's family, it was compounded by my great-grandfather being a habitual abusive drunk and adulterer who was frequently absent and eventually just walked out on the family. The eldest son ran away from home at age 14, found work and sent money back home. The eldest daughters raised the younger children, to the point of even having their own names for them (which always confused the hell out of me as a kid, because to their dying day they would refer to their siblings with their name for them).
You see the same thing happen with kids whose parent(s) are drug addicts. There was a good piece on NPR a few months back about a young girl and her two younger brothers whose mother was a complete wreck of a heroin addict. By the time the girl was 2 (yes, TWO), she was not only changing her brothers' diapers but also feeding them and watching them to keep them out of trouble. Kids can mature hella fast if the circumstances are dire enough.
They eventually ended up in a good foster home, but the story was mostly about how the girl basically had PTSD. It was proving very difficult for her to let go and let someone else be the parent, and for her to just be a young girl. Oh, and she was freaking out because their mother was pregnant again. >_<
I'm always surprised how protective my 4-year old daughter is of her 20-month old brother. If they're on a playground together, she watches out for other kids that might run into him and knock him down. And even though she's an introvert, she has no shyness about getting in another kid's face if he/she (usually he) shoves her brother out of the way or something. The sibling bond can be a powerful thing.