Hello and welcome! I am sorry you had to find us for advice for your brother and his wife.
Usually PE is diagnosed at 20 weeks at the earliest, but we've had some posters here who were diagnosed around 19 weeks or so. PE is diagnosed with blood pressure readings 140/90 or greater, measured on two separate occasions six hours apart and Proteinuria (protein in the urine) of >300.
I suspect your sister-in law is being sent to be seen by a high-risk OB (MFM, also called peri) to evaluate her further. If PE is suspected, they should check her BP, her urine for protein (a 24-hour urine collection, dips are not as accurate), her reflexes, and do a blood panel. Here is also a list of the most common symptoms: http://www.preeclampsia.org/signs-and-symptoms
They should also check the baby's growth.
Have your brother or sister-in law let you know why the doctors suspect preeclampsia?
I hope I could help with some basic info, but perhaps some others will chime in soon.
Let us know if you hear back from your family. Keeping my fingers crossed and sending along some hugs.
MC 3/2009 and 3/2011Henry
(1/1/2010-1/7/2010) - forever loved and missed; severe PE with Hellp; partial placental abruption, classical c-section at 25.6 weeksMatilda
(Nov. 2012, born at 35.4 weeks) - severe PE
Our pain has been put into words, placed into empty cradles, to remember that all our babies lived, that they mattered and always will. - Field of Cradles http://www.fieldofcradles.org/