Any Preeclampsia specialists here?

Not a medical person, but the link below gives a medical view of what the two doctors may have been thinking for this particular episode. Don't forget that this is still fiction, and even verified medical procedures known of that time period are not necessarily adhered to:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3964456/

Taken from the link:

"The weight of evidence suggests that Sybil is better off hospitalised. Sir Philip wrongly claims that toxaemia is rare. In his defence, though, his mismanagement is probably not all attributable to an inflated ego. Both doctors forget to use a sphygmomanometer (already available a decade earlier). As for Dr Clarkson, the family’s long-time trusted physician and friend, his argument that Sybil’s chance of survival at the hospital is ‘infinitesimal’ helps the family heal but contradicts his own earlier professional opinion, and may undermine trust with the family in the long run along with the lesson that needs to be learned by Sybil’s sisters, who themselves will be at higher risk of developing toxaemia of pregnancy."

It was probably inevitable, given the medical knowledge and medications available at the time to treat preeclampsia and ecclampsia (seizures that Sybil had). She likely was doomed to die.