Anger

Recently I've been interested in this guy that my cousin works with. While they were at work today, they decided to pull a "prank" on me by telling me that he had kissed my cousin. They both know how I feel about this guy, but they still did it. My cousin knows how much I hate it when people lie to me, and when I found out it was a prank, I was furious at both of them. My cousin says she was trying to prove a point to me.. What should I do?