pulmonary hypertension is merely the side effect of an underlying problem. doctors could try addressing the pulmonary hypertension problem, but if nobody pays attention to the underlying cause, then your sister will remain in that state.
here i believe is one good explanation as to why pulmonary hypertension occurs.
when a person has a lung problem, the blood vessels in their lungs constrict (narrows) therefore the blood being pumped by the right heart -- creates a back pressure. think of yourself as a water pump for example. you are pumping water through a tube with let's say 2 cm diameter... try pumping water through a 1cm diameter -- it's harder isn't it? and most of the fluids stay in that tubing -- being pumped back and forth -- creating a "back pressure" and this results in an increased blood pressure in the pulmonary artery -- thus results in pulmonary hypertension.
one of the most common cases wherein pulmonary hypertension would occur is among patients with pulmonary edema -- if your sister had this condition, wherein there is excess fluids in her blood, then a diuretic therapy would be advisable.
but if your sister has a lung problem wherein her blood vessels constricts (common case among patients with emphysema) then the doctors would need to address that underlying lung problem. she may be put to a mechanical ventilator so that her breathing will normalize and hopefully everything will be back to normal. otherwise, if the pressure keeps going up -- then there is a risk of right heart failure.
to sum it all up -- pulmonary hypertension is not a disease, but merely the side effect of an underlying problem which needs to address as soon as possible.