Yeah SJ it sounds like there was a placental issue with your sister's baby.

In a normal pg knots are not dangerous because the whartons jelly stops the veins and arteries being compressed. I have never heard of them being dangerous as such. Same with cord around the neck. Baby can't strangle because they don't breathe. It's only a problem if the cord is too short and baby can't be born (again your sister's story).