Future sister in-law bridesmaid

So one of my future sister in-laws is stressing me out.

She is a bridesmaid. I always thought we were on friendly and on good terms and she seemed genuinely excited when my fiancé proposed so I expected her to be more involved in the planning or at the very least not be difficult with wedding stuff.
My number one complaint is that she won't respond to any of my texts about the wedding. Bachelorette, buying the bridesmaid dress, etc. I have to ask my fiancé to follow up with her. And she replys to him immediately! She declined to go to the bachelorette (which is also mildly annoying but it's fine) but wouldn't respond to me or my MOH planning it. My fiancé had to ask and then she told him no. She never followed up with me either, no mention of declining the bachelorette. She did it again for the dress. I messaged her to buy the dress today because it is selling out. No response. I had to beg my fiancé to keep bothering her until she bought the dress! She even asked him about accessories. But still not a single message to me. (She also tried to buy the dress from a different website instead of Amazon because it was $5 cheaper. I told him to tell her that she can't do that in case the dresses aren't a true match)
Am I being too sensitive? Am I asking for too much? I don't want my fiancé to have to be our liaison but that's the way it's starting to look like it's going to have to be..