Clearly when there is no a pg in the womb but levels are rising it's an ectopic! how can a specialist miss that? The moment the u/s tech said there was nothing in the uterus, even before she found it in the tube, I knew. Plus I highly suspect your SIL's levels were rising but not every 48 hours. They are the main hints - low HCG for gestation, rising but too slowly, bleeding, pain and no pg found in the uterus.
I hope she does sue. It isn't just that she's lost her tube, it's the trauma.