Since pemphygus is an autoimmune disorder, I would first of all suggest making sure that your sister takes plenty of magnesium as magnesium deficiency has been strongly linked to immune/autoimmune disorders and it is estimated that anywhere from 70 to 95 percent of us are deficient in magnesium. Besides its many other essential benefits, magnesium is also a very good natural immune modulator. You can find out information about other natural immune modulators here:
Sugar is a blood vessel destroyer and is also unhealthy in many, many other ways - which explains why the World Health Organization has identified refined sugar as a number one cause of illness worldwide.
Cayenne pepper and vitamin C are both great for blood vessels. Both are high in antioxidants and both also help thin the blood.
Other good natural blood thinners include:
Dried fruits and berries, such as raisins, cranberries, prunes and cherries
Also, taking several digestive enzymes a few times daily on an empty stomach can help eliminate arterial plaque and dissolve blood clots.
Exercise is also among a natural blood thinner and it can be very helpful for conditions such as DVT
Compression stockings can also help relieve the symptoms of DVT
Another item well worth considering is a good blood tonic/blood cleanser. My favorite is the Jon Barron fromula Blood Support Tonic available from Baseline Nutritionals.