In addition to a normal internal circulatory system, they each have developed a second, external one connected to a second heart. The extra blood pumped through their bodies gives them incredible energy and endurance, but makes them far worse than any hemophiliac. To protect themselves they wear cloaks woven from spider silk, by spiders which crawl all over them, feeding on any mosquitoes drawn to the sisters. The cloaks cover their entire bodies aside from their mouths, and have a cape flowing off the back.
Their weapon of choice is a blade in tune with the phases of the moon. They appear simply as handles until held up to the moon, at which point the blade takes the shape of whatever phase the moon is currently in. In addition to slicing flesh and drawing blood, these blades have different effects for each subtle moon phase for a total of 28 effects. These include stripping away your memories of loved ones, your trust in others, your appreciation for music, and ability to digest food. Julian Parker knows at least most of them, possibly all of them, but not necessarily which effect goes with which phase. He definitely does not know which effect s caused by a waning half moon.
According to Julian Parker, the only method to defeat them involves a spray bottle full of sugar water.