This is DL’s fault and don’t let anyone tell you otherwise!
Well, I imagine it wasn't too long after they moved in together. One morning, while Yang and Blake were off at the library, Ruby decides to make Weiss some coffee, and well. Weiss has a morning icicle. Ruby's quite shocked, and Weiss is sure she's going to report it, but as days pass and Ruby behaves normally, Weiss is like "the hell?" and pulls Ruby aside about it. She's all "you haven't said anything?" and Ruby's all "Not even to Yang, which is a first." (part 2 to follow)