Just my 2 cents here. Spirulina and Chlorella belong to the Phylum Cholorophyta...in other words "chlorophyll producing seaweed". They are not land grasses. They contain many times the chlorophyll that you would see on land-dwelling plants. It is my understanding that chlorophyll has an oxidizing effect in the body. It is possible that your sister simply could not tolerate the high levels of chlorophyll. It is also possible that due to it's oxidation, she experienced a massive die-off or herxheimer reaction. A Herxheimer reaction occurs when large numbers of toxin-producing organisms suddenly die thereby releasing the stored toxins into the blood stream. Most parasites that reside in the human body can only survive in an anaerobic state...ie in the absence of oxygen. When the body is suddenly oxygenated...they die and release everything into the blood for you to deal with. This is often why with any cleanse, fast, flush, etc. you usually feel worse before you feel better.
That may be one explanation...as rambling as it is.
Seaweed in general is VERY GOOD FOOD. It has many trace minerals that we are missing in the modern American diet. In fact, it is a staple in Japan where they suffer a much lower incidence of many major diseases we experience in the US.
I can also tell you that as a student of Traditional Chinese Medicine, they do have a "cooling" effect on the body and that one of weak constitution should either not take them at all, or start out very slowly and build tolerance.