Just Received This & Your Advice Is Needed.

I love my husband. We trust each other. My husband has a younger brother to whom he is a best friend and a father figure, so they trust each other unconditionally, too.

Recently I’ve noticed the brother started to pay too much attention to me. Last week, he confessed he is in love with me and always was since he first saw me. He says he will tell my husband I cheated with him unless I actually sleep with him (the brother). I don’t know what to do. Do I tell my husband? On the other hand, if the brother lies about sleeping with me, which story will my husband believe – his brother’s, or my denial?
Or What do I do?