She's your sister, she'd be there for you more than anyone else. Cut her some slack.
Also because she's your sister, it'd be easier for you to be more straight with her. Just tell her like it is and maybe she'll get the hint? Some people need that slap in the face for them to move on. If she's a really sensitive girl then obviously approach it less harsh. Possibly get your mum/dad to do the talking? It sounds cowardly but I bet your mum or dad could make her feel better about it. That's what they're there for.