I agree with the earlier post about seeing a specialist - the pulmonologist.
If your sister is worried from the internet searching, you might want to reassure her that there is more info to be learned. It's easy to take a broad diagnosis of pulmonary hypertension and get freaked out when you start internet searching.
My sister had primary pulmonary hypertension (PPH), and it was awful, scary, and horrible. Words cannot describe how helpless I felt when the diagnosis was finally confirmed. PPH is ultra rare, so please know that your sister most likely does NOT have PPH.
Visiting the Pulmonary Hypertension Association website will probably help you more than any other website out there. I participated in a support group, and it was helpful for me as the caregiver. The website is http://www.phassociation.org
I'm not sure if my post has really helped or not. Please feel free to contact me privately if you'd like to talk further. I will do what I can to listen and encourage you.