Bridesmaid dress shopping question??

So I’m a bridesmaid in a wedding in the fall, and its my twin brother’s wedding so I will be in a ton of pictures. It will be the first big family event that my daughter will be at, and my parents and siblings are psyched for the pictures/everything to match.

I’m going bridesmaid dress shopping with my future sister in law in a few weeks, and while I’m still waiting for my results, I think I have a good chance at being pregnant this cycle. If not this cycle I will hopefully conceive in the next few months and be (visibly) pregnant in the fall. I have a young child already and have had several miscarriages this past year. For that reason, I really don’t want to say anything if I am pregnant. When we go dress shopping, I will only be 6-ish weeks if this cycle works out.

Should I say something to my future sister in law before we get the dresses? I haven’t told any of my sibling or my parents that we are TTC and no one in my family knows about the miscarriages. I don’t want to tell them but I also don’t want to have my future sister in law hate me.

Would it be horrible to just not say anything and buy the dress, and then deal with it after the 1st trimester if things work out? What would you do/have you done in this situation?