Our typefaces often reference each other, developing themes others have started. The oblique drawing of Elephant was originally designed as an alternate roman (ie vertical) typeface with features based on a true italic, with typical italic features such as a one story a, and a softening of the rigid straight line theme. When used next to the regular version the difference wasn’t defined enough, but the idea of two typefaces with a very different character shape but based around the same grid was interesting enough to develop to a full font, which became Sister.