DD1 carries DS1 all the time. She's a very strong almost 7 year old and he's a heavy 15 month old. She loves it. But if he gets too heavy or squirmy she puts him down very carefully. I also agree that in so many parts of the world this is completely normal. I was the youngest of 4 and my oldest brother who was 8 when I was born got me up in the morning (once I was 8 or 9 months old), carried me downstairs, got me breakfast etc.
As for developing spines, my DD would never carry DS long enough for it to hurt (not at all like being pregnant). My DD was desperate to help when DS came along and I figured I was better off showing her the safe way to do things rather than her doing it behind my back.
Why should the kid be lumped with looking after the baby while the mum is off (having a latte?).
For me personally, I would be comfortable with my DD using a supportive carrier over a short period of time to carry an older baby if she asked. But this would be when she's a lot older (8+) anything younger than that I would be concerned over her spacial awareness to judge where to move with the baby attached.
Keeping in mind we don't know the age of the girl or the weight of the baby or really, how far away the mother really was in the OP - its too hard to judge her actions as wrong or right.
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