Help with movie

All I remember is a woman kills her husband she gets away with it and she has a sister. The sister gets told by her older sister where she hid the gun. The sister goes to the police and tells them what she heard. The police ask her to wear a wire she does but on the day the sister wears a wire the killer says something diffirent. The only reason why the police find out where the gun is they are moving some items out of the house. The police move one flower box but they can't move the other. Its been filled with concrete. Then they find the gun in the concere.

I think it's based on a real story.