Am I wrong for being upset?

So my future SIL got engaged last month. When she did she told everyone she wanted to me married in September. My fiance asked her to not pick one this one particular day because that's the day of my white coat ceremony. I'm starting medical school and this is the ceremony where we get our first white doctors coat. Its a huge deal for me as I've been working on getting into med school for years! My mum is unable to attend as she is currently unable to travel for health reasons and my dad passed a few years ago. So my fiance is the only person able to attend (I don't have any other living family besides him and my mum). I know it's not as big as convocation and I'm not a doctor yet, but this is the first big step towards that.

Well, my future SIL said this was fine as she'd most likely get married on a Saturday and my ceremony is on a Wednesday. She just sent out the dates for her wedding and guess what? Its the same day (and even starts at the same time) as my white coat ceremony. My future SIL and I have never really liked each other and I always felt like shed do things just to upset me but I was never sure and always brushed it off and ignored it. Now I feel like she's doing this just to hurt me. The wedding is in her own backyard and she's having her future BIL officiate. The wedding is family only and the food is all being made by her mother. There are no caterers, band/DJ or anything like that. So it really can't be that this is the only day (and time) in the whole month of September where it works. Since it is a Wednesday it is actually difficult for some other guests to even attend due to work.

I know it's her wedding and I want her brother to be there with her. But I want him with me too. I know that a wedding is more important than me getting my white coat so I've told him he should go and I'll see if a friend (I don't have friends in the city where my med school is as I'll be moving there next month and my friends do need to work so taking time off in the middle of the week won't be easy) can take his place. I'm just crushed that I'll likely be alone on that day. My fiance said he's going to talk to his sister and see if she'll move the day, but I know that I wouldn't want someone to do that about my wedding day. I've told him not too, but he's angry that she picked the day and has said he's not letting me go alone. I just feel crushed that she'd pick this one day, but I'm going to be a bride too and wouldn't want someone to try to mess with my day so now I feel horrible for being upset with her. But, then I think she's doing this on purpose so maybe I do have the right to be upset?