Most Helpful Guy
You didn't say how old your sister is, and it matters.
If those of you who love your sister know her fiance is bad, then let her know she is doing the right thing.
My brother married an idiot liberal-Socialist (not the smart kind). No one in the family liked her and we couldn't understand why he would. Many years later, he announced they were getting a divorce. All of us cheered. He has been dating several women since then and we like all of them.
Most Helpful Girl
Here's what you do. Keep her busy so that she can keep her mind off of the broken engagement. Get involved in hobbies together, now is the time to really get some sister bonding time :)
Have movie nights, call to check in on her and offer her to come stay with you if needed.
Help her move out of the joint house (if there is one). Offer to send out an email or call people who were invited if they already invited people to the wedding, and then let them know it has been called off. Basically do what you can to take some of the pressure off of her.
She will still love him of course. But just as she fell in love with this guy, she will fall in love again :)
Be supportive of her and whatever her needs are. Go get mani/pedi's together, do things that are fun and keep her mind off of the broken engagement.