My boyfriend lied about having a brother?

At the beginning of our relationship (3 months ago), my boyfriend told me he has a brother. He even told me a very long story about him. Today, when I was talking about my brother, my boyfriend said he wished he had siblings too. When I told him that he previously told me he had a brother, he completely denied it. I was so confused and shocked that I couldn't say anything. Why would someone lie about such thing? What would you do if you were in my position?
My boyfriend lied about having a brother?
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