Helped relative to marry, now he is threatening to spoil my career

I supported my uncle marry a girl who is of other religion. I was not the witness in their marriage and not present in any of their ceremony. What mistake I did was , booked a hotel room for them where they never showed. And I supported him , adviced him in every where. As he was close to me. Now he is threatening that he will give proofs and spoil my career. And kill everyone who comes in between him and her. The problem arised when girls family do not accepted him and girl is also now against him. The girls is my mothers sisters daughter and the boy is my grand fathers brothers son. Boy is muslim and girl is hindu. Please help me. What should I do. My parents are supporting girl so he is threatening my parents that he will destroy my career.