AITA for being upset because my brothers fiancé tried on my grandmothers wedding dress?

I will be changing names in this story to remain anonymous. I apologize in advance for the long post. I promise it’s important to the story line.

I (20 F) got engaged almost two weeks ago to my wonderful fiancé (23M) John. My brother, Logan, (23 M) proposed to his gf last weekend, Sarah (20 F). When my fiancé proposed, we had everybody there with us. My parents, Logan, Sarah, and even got my uncle who lives 10 hours away to take pictures. It was in front of a waterfall, he took me away towards the waterfall so it would still be intimate but I still had my family there, it was perfect. It is important to know that me and him got engaged first and me and him started dating a little over 4 months before my brother and his fiancé started dating.

The whole rest of the day while we were going to different places taking pictures, my fiancé kept mentioning that Sarah had this look on her face as if she was thinking “well Logan is older and he should be the one getting engaged and married first”. I didn’t really think anything of it, because in all honestly I haven’t really liked her that much from the beginning. She would make passive aggressive comments to me, look like she was jealous for me having a good relationship with my brother, and basically treat my brother like he was her pet rather than a boyfriend. I had said multiple things to my parents about her because we are a pretty tight knit family and if there is an issue within our family, we always sit down and talk about it. Or so I thought. When I told my parents how she had been treating me, they just made excuses for it saying “she had a hard childhood” or “she just doesn’t know any better” which is not true because she grew up in a family pretty similar to mine and even if she didn’t, that’s no excuse. Eventually, I decided to let it go, because it seemed like she had changed some over the year that her and my brother had been together. Until this past weekend.

The day after me and John got engaged, my brother asked me to drive 6 hours to where my grandmother lives and where he is doing his internship to photograph him proposing to Sarah. He was very polite enough to ask me first if it was okay for them to get engaged a week after me and John did. To which I told him, “of course I don’t care! You propose to her whenever you want to, me and John have nothing to do with when y’all want to get engaged”.

So, that Wednesday I drove 6 hours up to my grandmas house, stayed with her until Thursday, and then moved all of my things to my aunts house to stay with her for Friday night while Sarah and Logan stayed with my grandma. Then, moved all of my stuff back over to my grandmas for Saturday night. Sunday afternoon was when he proposed. And let me tell you, I went all out for this. I put on a disguise so she wouldn’t know who I was and scoped out a beautiful place at the location Logan asked me to before they got there. Which, mind you, was an hour away from where my grandma lives. It was a beautiful proposal, and she loved the disguise. When we finished taking pictures, we all went back to my grandmas house to see her and take more pictures over there.

The next day, my mom came into town and everything was great. We went to the lake and tanned and played in the water. This is where it started to go downhill. As we were tanning on the beach, Sarah started asking about mine and John’s wedding plans. I told her that we have a couple dates in mind, and we are working out some kinks in how long the ceremony and reception will be. I told her that John comes from a very nontraditional family and he wanted a very quick wedding with an hour reception. I told him that I wanted a 15-30 min ceremony with a 5 hour max reception and we ended up settling on a 4 hour reception. She then says “it seems like John is pretty stubborn, isn’t he?” In a very passive aggressive tone. I stated “he is but so am I but we usually come to a middle ground pretty easily.” We then got on the subject of how often we fight. I told her that me and John rarely fight and that we didn’t really have a true argument until about a year into our relationship and it wasn’t even a “fight” it was more of a disagreement. My brother then said “yea it was pretty rocky there for us for a while because she wanted to get engaged and we were doing long distance and I have to finish my internship before we get married. But we were able to work it all out.” She then got this exasperated look on her face and said pretty quickly “well it wasn’t rocky at all, like our relationship wasn’t in danger, we were just arguing here and there” and then shot my brother a look. Which I didn’t like but I brushed off. There were some other things she said and did, but I won’t get into it.

The next day, I had to leave and go back home, but I woke up late because I had to sleep on a couch in the basement with my brother in another bed because my mom took the bed I was sleeping on and Sarah was in the other bedroom. (My family does not believe in staying in the same rooms as each other before we are married.). So I wake up and I walk upstairs, and I see Sarah wearing my grandmothers wedding dress and everyone fawning over her. I immediately had to hold back tears and blowing up. I have a feeling that it was my grandmothers idea to have her try it on, but I don’t blame my grandmother, because she can be ditsy and doesn’t really think about those things. Sarah, however, I feel like should have thought about that knowing that I got engaged first and she isn’t even blood related to my grandmother. And the fact that I hadn’t even looked at it or tried it on at all. I just feel like I was robbed of a pretty important moment between me, my mother, and my grandmother.

The reason I didn’t say anything to her was because I did not want to become the a hole in the situation because I knew it would all come back on me and I would be labeled as being “too emotional” or “too much”. So I need to know, am I the a hole?