I suppose such dogs that can be trusted with your life are rare though.
If your sister had to escape her play pen to pester the dog it sounds like your mother was smart enough to keep them separated when your sister was very small. The problem I have is many people now have babies and toddlers and large dogs interacting with little or no adult supervision- as to how the child treats the dogs or what a dog’s instinct might be to pain or fear.
I think what you described is what most dogs do when small children pester them, most don’t lash out at the child. Also as you described it your mother kept the separation between baby/toddler and large dog so your sister was not allowed to pester that dog on a hourly/daily basis which might have made a difference. Another point is had the dog reacted badly it would have been a big deal just due to size difference.