There is a key difference between feeling love for somebody, and just simply feeling an attraction toward somebody. In your case, it sounds like you are simply finding your boyfriend's brother to be attractive. Everybody has a tendency to feel attracted toward another person, it's 100% normal. However, if in this case you MIGHT feel that you are getting a really strong emotional connection with your boyfriend's brother, you should avoid breaking up with your boyfriend and getting with his brother. That would be a very selfish thing to do, it could possibly crumble your boyfriend's relationship with his brother. If you had enough respect for your current boyfriend, you wouldn't even dare to do such a thing.
Don't show any signs to your boyfriend that you have an attraction to his brother. Keep going strong with your relationship, and I hope everything works out just fine.
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I'm from Africa, BITCH!