Moh/sister Issues

To no surprise of my own my MOH and sister has done it once again. She announced this past week that she will be getting married this July! A destination wedding, a few states away..on 4th of July weekend. (She had yet to tell my parents, although we are all very close & she expects them to pay for part of the event). This is just a month before my wedding. (That I was then helping plan). My sister is not engaged and has only been exclusively dating this man for about 3 months despite him not honking they've been exclusive for closer to 7 months (the family met him only about 6 weeks ago). I tried describe to her that I was hurt by her actions, and that she has made a selfish decision to which she responded that this is not a competition..although I've been engaged for months and have a wedding planned already.


I told her that I support her relationship and her upcoming marriage; however, I am confused as to why it must be at this moment in time. She had no conclusive answer. I told her regardless I would get through it and we'd both end up happy.
Two days later she texted me and told me that her boyfriend no longer wants to have the wedding in July.
Now..approximately 3 days later she has texted me and asked me to go wedding dress shopping with her on a date in May that I already have plans. I asked her what her wedding plans are and she refused to tell me.
I'm frustrated that she is seemingly orchestrating another life event of hers to make the spotlight on her, when she in fact is not engaged at this time and she has not been able to express her commitment to this man. While I'm totally accepting of a quick engagement and knowing what you want, and seizing the moment, I know that she is trying to take away.
(She had introduced him to the family the day I tried on wedding dresses, too).