I've looked at the family tree, and I've noticed something. There is a lack of of female relitives. So, I've decided to think of some ladies to add to the family. Hopefully, they'll catch on, but only time will tell.
Die Großdame is the wife of Der Großmann, and the mother of Splendor man. Unlike her husband and second child, Slenderman, she is a kindred forest spirit, and only kills when necessary. She is where Splendor Man and Tender Man get their kind demeanor from.
Artemis Mann, More commonly known as the Thin Lady, is Slender Man's twin sister, with a similar appearence, but with a pencil skirt and jack boots, and her mother's platinum blonde hair. Like her brother, she suffers from Middle Child Syndrome, but is shy, creative, and more introversial, as opposed to her miscreant twin brother. She can be likened to a wart or mole; benign most of the time, but can turn nasty. She can grow appendages like her brother, but prefers a pair of bird-like wings, rather than extra arms or tentacles.
Trendy's little sister (or Suzie, if you will)
As her tittle suggests, she is the younger sister of Trender Man, currently going through a 'rebelious teenager' phase. She has her own all-girl rock band, called "Return to Slender" and she often goes by the nickname "Suzie Slender" or "Lil' Slender Gal" amongst her friends and bandmembers, due to her relation to the ever-famous Slender Man. Like her father, she has a New York accent, but not as noticable.