My friend was dumped for my sister 4 months ago, but is crazy over the boy. she's annoying all; what can I do?

A friend of mine (we'll call her Sally), has liked this boy (we'll call him James) for about a year and a half. He went out with her for 3 months this year to tolerate her (she can be incredibly annoying and dramatic) but everyone except her could tell he didn't like her. He never really liked her, and broke up with her because he still liked my sister (they dated about 2 years ago), and asked her out. She said yes, and they are in love with each other, and are amazingly cute together, but Sally cannot get over James, even though they broke up in February. Sally is extremely jealous and spiteful, and told James she still likes him. He told her he didnt feel the same way. A mutual friend of mine and Sally's told me that Sally said she "has a plan" to get James back from my sister; everyone knows this will not happen. As you can see, I am in an awful position. What could I say to Sally that will make her realize she cannot act like this and needs to move on (without being overly mean)?