My Brothers Fiance ignores me. What could be the reason and what can I do?

Since my brother got engaged his fiance ignores me and won't talk to me.

I be nice to her and try make conversation with her. When she thinks the conversation is finshed she will then talk to someone else.

My brother tries to get her to talk to me. When he does this she turns on him and starts giving out to him.

I have asked my brother why is she like this. He told me many times diferent answers like she is shy, she doesn't know what to say and many other answers.

Once he told me she is shy and a few minutes later i heard him talk to our parents saying that she is bubbly and will talk to everybody.

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I want to be able to be friends with her but she is making it difficult. Before they got engaged she used to be really nice and friendly to me. She used to talk to me then. I was so excited when i found out my brother was proposing to her. He let me help him get the things he needed for when he proposed.
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She is having a small party for her birthday next month and we are all invuted. I dont know whether to go or not. Either go and put her nose out of joint or give her the satisfaction of me not being there. When i can't go to family outings and she is there, she said that i am always sick when we have family outings. Thats not fair as i can't help it if i am sick.
My Brothers Fiance ignores me. What could be the reason and what can I do?
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