What should I do?

Kill that guy. With a machine gun.
Or,try to cheer her up,and possibly give her advice..

Tell her that it will be okay. Heartbreak is difficult, but it's not impossible to get over it. It will take time, but eventually her ハート, 心 will heal. Promise her that. Comfort is the best thing あなた could possibly offer your sister right now.

Remind her that obviously he was bad for her life. Tell her that this means there's an even better guy waiting for her out there.

One day, she'll wake up, and she won't 愛 him anymore. Tell her to not worry.. about the people in her past, because there's a reason they didn't make it to her future.

Do some of her お気に入り things with her it will make her fell better

Tell her he wasnt worth her time! That he was just a bump in the road and she has to 移動する on. Then have a movie night and eat ジャンク 食 and just have a girls nite with her! No boys no drama! ^-^

Just be there for her.

Well... I don't know what your sister's like, but here's what I always do when my フレンズ are sad about a break-up:

1) Call the man some funny insults. (i.e. my friend Kamrin was upset, so I called this guy that was insulting her a blood-belching vagina and a chuckle-fuck.)

2) Just comfort her and assure her that there will be other guys that aren't total jack-asses.

3) Hang out with her, try to lighten the mood.

4) Kill the boy if all else fails :D