These stories are so sad. OP, my story is very similar to yours. My dad's sister died of pneumonia when she was 11, on Boxing Day. Dad was 14 and was sent out to find the doctor. The doctor was not at home, and Dad returned to find his sister had died.
It basically wrecked his relationship with his parents for years afterwards - he still hasn't spoken a great deal about it, but I can only guess at their grief, and how they just didn't know how to deal with an adolescent boy who'd lost his sister.
My nan (her mum) died 18 months ago. There was one photo of my aunt that she always had out, that was taken the summer before she died. That's the only picture I had ever seen of her. In nan's things, Dad found a gorgeous photograph of all four of them when his sister was about three, and he put it on the back of the Order of Service. They look sooooo happy. It made me cry and cry and cry to think how all of them would be affected by what happened.
And again, I think it unlikely she would have died in this day and age. Reading between the lines (and the family genes), I think she was very likely asthmatic, and caught a winter bug that went to her chest.
Crying again now, so no, OP, yanbu.