My sister doesn't want me in her bridal party. Do I have a right to be mad?

A few months ago I found out my sister got engaged and I was thrilled for her. I really like her fiance and I knew that she had wanted him to propose for quite some time now. So since then my sister has been planning her wedding and I have been helping her because I umed since I was her only sister that I would be a bridesmaid. Well I definitely umed wrong, because yesterday she informed me that she already picked her bridesmaids and I was not one of them. She said the reason is because her fiance's been pressuring her to ask his two sisters and she has already picked 3 of her friends that she is very close with to be bridesmaids as well. For reasons only known to her she can not have more than 5 bridesmaids. When she told me this I admit I let my emotions get the best of me and got really upset, which led her to say that I was being selfish and needed to grow up. Its true that we're not super close, but I'm still her sister! Is this not a betrayal? Because thats what it feels like. I would never dream of not having my sister stand up there with me at my wedding. The fact that she would have her fiance's sister's up there and not me is what really gets my blood boiling. Would it be wrong to not attend the wedding because of this? The wedding is not until August, but unless my sister changes her mind and lets me be a bridesmaid I definitely don't see myself forgiving her for this anytime soon.