I am in love with my brother's ex-girlfriend of 2 years. Over those 2 years, we

I am in love with my brother's ex-girlfriend of 2 years. Over those 2 years, we became best friends and I developed feelings for her. My question is, now that my brother and her are no longer together, is MORALLY wrong to start a relationship with her? Here is what I have considered: From what I have learned about objective morality/ethics I could follow the Golden Rule "Treat other as you would want to be treated". I have dismissed this on the basis that yes, if I were my brother I would be annoyed by my brother dating my ex, but I would also want my brother to be happy and, after weighing everything on both sides, I would concede to allowing my brother to do what makes him happy. If I take an egoistic approach, I probably wouldn't be asking this question because I would do what is best for me. If I take a utilitarian approach I would consider everyone I am affecting equally, and do what is best for the majority and in that case, I would harm one person (my brother) and do what's best for the majority (me and her). So far, all I have gotten from people are opinions which range from "do what your heart tells you" to "That's wrong". Have not received a reason as to why it is wrong. P.S.: (1) I know she is not the only girl in the world that I can fall in love with, but fact is I am in love with her now and she is my best friend, (2) I don't want opinions, I want a reason as to WHY it is wrong that is not biased and that is based on some fundamental, objective moral standard, and (3) I am fully aware of the implications with my family and hers, I know their opinions on the matter (my brother personally says he doesn't care, but that he would be disappointed, but offered no reason for his disappointment). Thank you.

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