If Janni had killed Cynthia, then it's an elimination game between Janni & Jimmy. Whoever kills the other gets the money. Since Janni had her, he can kill her anytime. But, makes it difficult for Jimmy.

Also, adding to what I just wrote, perhaps they were keeping her alive in case Jimmy wanted Cynthia back?

No. Jimmy didn't give a crap about her. I never thought about this until you mentioned it. lol but I still love this movie one of my favorite comedies ever.
I like to think that Yanni kept her alive out of arrogance; he wanted to tap her (whack her haha) after killing Jimmy. Yanni wasn't concerned about her because he could do her in whenever he wanted. That's the only explanation I can think of.

From what I understand, Janni kept Cynthia alive as a bit of insurance. Jimmy still cared for her and would come directly after Janni if he knew she was dead.
Sticking feathers up your ass does not make you a chicken~Tyler Durden