What should someone do in this situation?

Many people feel that a relationship break-up only affects the people involved. These people forget the third parties. In this situation, the third party is you. You are now beginning to feel you must intervene and help her.

This situation is extremely difficult to judge. What most people try to do, is get the 'breakee' (the victim) to hate the 'breaker', but as you said, she still loves him which creates a problem.

The best thing to do is talk very general. "I know it is hard to hear, but people move on. Just look at yourself. Look at what you have accomplished recently (describe something good your sister has done). This was a great step forward. You did it by yourself."

Then explain the situation to her. This sounds silly, but because of her overwhelming feelings, she may actually be confused. "What you must remember is that HE (place emphasis) broke up with you. HE chose to leave. HE made the wrong decision. HE lost you. (If you know details about negative parts in their relationship, say them now). All you can do now, is forget about him. Look forward to things coming in your life (describe positive things that will soon be happening to her, things for her to look forward to).

Try to end your 'speech' away from the topic of him. Your aim here is to divert her thinking away from him on to looking forward to future occasions.

As hard as it is, tell her to cut contact with her ex

not only have they broken up, but to be friends when she still has feelings for him makes any friendship difficult.

Every time he gets a girlfriend - it will hurt her

She needs to move on, and to do so, she should cut contact with her ex so she has time to heal

Forget about him and move on! Your my sister and you like me so you will have plenty if luck with any guy who walks by you!

Hit her ex with a sweet chin music Heartbreak. Then tell her ex to comfort her and if he doesn't, hit him with another sweet chin music.