I disagree with Grandmatobe. If they are together, living together, sharing household responsibilities, and going to get married soon, as partners and soon to be husband and wife, then yes, she should know, especially if she "would not have a problem with it", as he said, but It is not your place to tell her. That's your brother's business. Discuss this with him and how greatful, but uncomfortable it has made you. Why start a marriage with secrets?