What would you do if you were in the situation i am now ?

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My senior brother fell in love with a nice woman last year. Few months after they met, they decided to live together in my brother apartment. They really love each other, and my family likes her well. She is a fine, kind and good woman for my brother. She told me she would like to be my brother's wife. Recently, my brother told me with joy that he is organizing a meeting in the house of his girlfriend parents to ask her in marriage in the presence of her parents. A very close friend of mine use to tell me that he got a new girlfriend who is so kind,sweet on bed, and used to buy him some gifts. He is not the kind of guys who wants to get marry or who want to have only one partner. But he told me that this woman might make him change his mind soon to have only one partner. My friend visited me two days ago, and told me again about his same girlfriend and showed me their recent pictures while kissing on bed in his bedroom. I was shocked to see that it was my senior brother girlfriend kissing my best friend on his bed !!! The way i reacted he asked if i knew her. Then i answered him that she is my senior brother girlfriend and that my brother is planning to marry her. He was also surprised and shocked, and asked me not to tell it to my senior brother. Today my brother visited me and was so happy. He asked me to take part of the meeting in the house of his girlfriend parents. I answered him that i am sorry i can not because i have an important business meeting on that time. I lied to him. I don't know what to do. My brother loves her so much, and they look like the best couple i ever seen in my family. I don't know how long i can keep that truth in me. Now it's hard for me to look at him in the face. I wonder what to do now. I hope your wise advises will help me face this situation.