Your sister had brain haemorrhage. Brain haemorrhage occurs due to many causes. Three chief causes are:
Rupture of arteries, mostly due to uncontrolled hypertension.
Rupture of abnormal twigs (usually developmental) of arteries and veins (called AV malformations).
Rupture of small berry like outpouchings from arteries called aneurysms.
Occasionally, in females (especially during pregnancy) the veins of the brain or their draining areas (called venous sinuses) can get blocked resulting in haemorrhages and areas of infraction. From the brief description that you have provided, it seems that your sister had brain haemorrhage due to the first cause.
The effect of haemorrhage depends on location and side of the brain that is affected. I am not sure what kind of surgery is planned for her. Usually the surgery is reserved for the last two types mentioned above (AV malformations and aneurysms).