Unusual, but not unheard of. Hot, petite female friend of mine gets along extremely well with her older brother, and they look alike. She hugs him, and he’ll wrap his arm around her waist, pull her in to talk and joke, and they do look like a couple. The way she talks about and worships him, even said a few times about him being “in such good shape..should see his abs” only adds to the thinking that they’ve had sex. Makes a lot of guys wish they were her brother, too. I’m one of them.