Dear LIB Readers: Should I Tell Him?

My immediate younger brother is about to get engaged to a lady I had a one night stand with in May. I met her at a club, took her home, had sex with her, gave her money and put her in a taxi. I was introduced to this same lady last week as my brother's girlfriend. He told me in confidence he plans to marry her. I feel I should say something to him. Could you help discuss this on your forum dearest Linda.

Dearest readers, what do you think, should he speak or shut the f up?